How Best Value works: its own sheet, opened from the Key sheet and More info sheet

The explanation used to be a section inside the Key sheet. It is a standalone
HowBestValueSheet now, opened from a tappable row with a chevron: at the bottom of
the Key sheet, and at the bottom of the More info sheet below the Best Value card.

Gating: the More info row sits INSIDE the same `if let fuelSavings` as the card it
follows, so it disappears with the feature. Verified with Best Value toggled off in
Settings: the More info sheet then shows neither the card nor the row. The Key
sheet row is deliberately ungated — that sheet is the legend and is reachable
whatever the feature is doing.

Copy is unchanged: the same Localizable.strings strings are reused, and the old
section header became the sheet's title. A DEBUG-only `-showBestValueInfo` hook
opens the sheet without a tap.

FuelBoard/ContentView.swift

@@ -566,10 +566,34 @@ struct ContentView: View {
         func source(_ basis: FuelSavingsDistanceBasis) -> String {
             basis == .roadRoute ? "road route" : "straight-line estimate"
         }
-        let price = station.prices[selectedFuel].map { String(format: "%.1fp/L", $0) } ?? "—"
-        let baselinePrice = baseline?.prices[selectedFuel].map { String(format: "%.1fp/L", $0) } ?? "—"
+        func amount(_ value: FuelSavingsFillAmount) -> String {
+            switch value.unit {
+            case .pounds: return String(format: "£%.2f", value.value)
+            case .litres: return String(format: "%.2f L", value.value)
+            }
+        }
+        func rawMoney(_ value: Double?) -> String {
+            value.map { String(format: "£%.6f", $0) } ?? "—"
+        }
+        let candidatePriceValue = station.prices[selectedFuel]
+        let baselinePriceValue = baseline?.prices[selectedFuel]
+        let price = candidatePriceValue.map { String(format: "%.1fp/L", $0) } ?? "—"
+        let baselinePrice = baselinePriceValue.map { String(format: "%.1fp/L", $0) } ?? "—"
+        let priceDifference = candidatePriceValue.flatMap { candidate in
+            baselinePriceValue.map { String(format: "%+.1fp/L", $0 - candidate) }
+        } ?? "—"
+        let litresBasis = sessionFuelSavingsAmount.unit == .pounds ? "candidate price" : "fixed litres"
         let rows: [(String, String)] = [
             ("Station", "\(station.name) (\(station.id))"),
+            ("Settings default fill-up", amount(fuelSavingsSettings.preferredAmount)),
+            ("Effective/session fill-up", amount(sessionFuelSavingsAmount)),
+            ("Fill-up unit", sessionFuelSavingsAmount.unit.displayName),
+            ("MPG", String(format: "%.1f UK MPG", fuelSavingsSettings.mpg)),
+            ("Candidate price", price),
+            ("Baseline price", baselinePrice),
+            ("Price difference", priceDifference),
+            ("Candidate litres", result.map { String(format: "%.6f L · %@", $0.fillLitres, litresBasis) } ?? "—"),
+            ("Gross saving raw", rawMoney(result?.grossSavingGBP)),
             ("Displayed distance", "\(distanceUnit.format(stationDistance.displayDistanceKM)) · \(source(stationDistance.displaySource))"),
             ("Outward road", meters(stationDistance.outwardMeters)),
             ("Return road", stationDistance.hasMeasuredReturn ? meters(stationDistance.returnMeters) : "inferred or unavailable"),
@@ -581,12 +605,11 @@ struct ContentView: View {
             ("Station journey", miles(journey?.miles)),
             ("Baseline journey", miles(baselineJourney?.miles)),
             ("Additional journey", miles(result?.extraDistanceMiles)),
+            ("Additional fuel used", result.map { String(format: "%.6f L", $0.extraFuelLitres) } ?? "—"),
             ("Confidence", result?.journeyConfidence.rawValue ?? "estimated"),
-            ("Fuel price", price),
-            ("Baseline price", baselinePrice),
-            ("Gross saving", result.map { FuelSavingsFormat.money($0.grossSavingGBP) } ?? "—"),
-            ("Travel cost", result.map { FuelSavingsFormat.money($0.extraDrivingCostGBP) } ?? "—"),
-            ("Net saving", result.map { FuelSavingsFormat.money($0.netSavingGBP) } ?? "—")
+            ("Travel cost raw", rawMoney(result?.extraDrivingCostGBP)),
+            ("Net saving raw", rawMoney(result?.netSavingGBP)),
+            ("Displayed net saving", result?.headline() ?? "—")
         ]
         return FuelSavingsJourneyDiagnostics(rows: rows)
         #else
@@ -1104,6 +1127,13 @@ struct ContentView: View {
             ensureBestValueAlertModeIsAvailable()
             updateAlertMonitor()
         }
+        .onChange(of: sessionFuelSavingsAmount) { _, _ in
+            // This remains a session-only override: do not write it to FuelStore.
+            // Any active Best Value alert/Live Activity must nevertheless use
+            // the same effective amount as rows, details, favourites and sorting.
+            updateAlertMonitor()
+            updateLiveActivity()
+        }
         .onChange(of: scenePhase) { _, newPhase in
             if newPhase == .active {
                 // Never start location tracking while onboarding is on screen —

FuelBoard/FuelSavingsViews.swift

@@ -119,6 +119,9 @@ struct FuelSavingsSummaryView: View {
                 VStack(alignment: .leading, spacing: 10) {
                     capsule(for: result, compact: false)
                         .accessibilityLabel(accessibilityHeadline(for: result))
+                    LabeledContent("Your fill-up", value: amountText)
+                    LabeledContent("Fuel you'll receive", value: String(format: "%.2f litres", result.fillLitres))
+                        .monospacedDigit()
                     if let extraTravel = FuelSavingsFormat.extraTravelLabel(
                         extraDistanceMiles: result.extraDistanceMiles,
                         unit: distanceUnit,
@@ -134,9 +137,26 @@ struct FuelSavingsSummaryView: View {
                             .font(.caption)
                             .foregroundStyle(.secondary)
                     } else {
-                        Text("Compared with \(baselineStationName ?? "your nearest station") for \(amountText).").font(.caption).foregroundStyle(.secondary)
-                        LabeledContent("Fuel-price saving", value: FuelSavingsFormat.money(result.grossSavingGBP))
+                        Text("Compared with \(baselineStationName ?? "your nearest station").")
+                            .font(.caption)
+                            .foregroundStyle(.secondary)
+                        LabeledContent(
+                            result.grossSavingGBP >= 0 ? "Fuel-price saving" : "Fuel-price difference",
+                            value: result.grossSavingGBP >= 0
+                                ? FuelSavingsFormat.money(result.grossSavingGBP)
+                                : "Costs \(FuelSavingsFormat.money(abs(result.grossSavingGBP))) more"
+                        )
                         LabeledContent("Extra driving cost", value: FuelSavingsFormat.money(result.extraDrivingCostGBP))
+                        Divider()
+                        LabeledContent {
+                            Text(result.headline())
+                                .fontWeight(.semibold)
+                                .foregroundStyle(capsuleColor(for: result.status))
+                                .monospacedDigit()
+                        } label: {
+                            Text("Total")
+                                .fontWeight(.semibold)
+                        }
                         if result.journeyConfidence.isEstimated {
                             Text("Estimated from straight-line or incomplete route data — refined once both routes are measured.")
                                 .font(.caption2)

FuelBoard/HowBestValueSheet.swift

@@ -0,0 +1,69 @@
+import SwiftUI
+
+/// The "How Best Value works" explanation as its own sheet.
+///
+/// It used to sit inline in the Key sheet as a `Section`; it is one shared view
+/// now so the Key sheet and the More info sheet can both open it, and the copy is
+/// unchanged (the same `Localizable.strings` keys are reused — the old section
+/// header string is the sheet's title).
+struct HowBestValueSheet: View {
+    @Environment(\.dismiss) private var dismiss
+
+    var body: some View {
+        NavigationStack {
+            List {
+                Section {
+                    VStack(alignment: .leading, spacing: 10) {
+                        Text("Best Value compares each station with the closest one. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.")
+                        Text("Your fill-up amount and MPG are used to show whether you could save overall.")
+                        Text("Results are marked “Approx.” until road distances are available.")
+                            .foregroundStyle(.secondary)
+                    }
+                    .font(.subheadline)
+                    .padding(.vertical, 4)
+                }
+            }
+            .navigationTitle("How Best Value works")
+            .navigationBarTitleDisplayMode(.inline)
+            .toolbar {
+                ToolbarItem(placement: .confirmationAction) {
+                    Button("Done") { dismiss() }
+                }
+            }
+        }
+        .presentationDetents([.fraction(0.42)])
+        .presentationBackground(Color(UIColor.systemGroupedBackground))
+    }
+}
+
+/// The row that opens `HowBestValueSheet`. A tappable row with a chevron, like
+/// the other rows in both sheets. `.buttonStyle(.plain)` keeps it from becoming a
+/// row-wide tap target that could swallow a neighbouring control (see the
+/// two-buttons-in-one-row rule).
+struct HowBestValueRow: View {
+    let action: () -> Void
+
+    var body: some View {
+        Button(action: action) {
+            HStack(spacing: 8) {
+                Text("How Best Value works")
+                Spacer(minLength: 8)
+                Image(systemName: "chevron.forward")
+                    .font(.footnote.weight(.semibold))
+                    .foregroundStyle(.tertiary)
+            }
+        }
+        .buttonStyle(.plain)
+    }
+}
+
+#if DEBUG
+/// QA hook support: `-showBestValueInfo` opens this sheet without a tap, so it can
+/// be screenshotted (simctl has no tap command) — the same pattern as
+/// `-showKeySheet` / `-showDetail`.
+enum BestValueInfoQA {
+    static var isRequested: Bool {
+        ProcessInfo.processInfo.arguments.contains("-showBestValueInfo")
+    }
+}
+#endif

FuelBoard/ProximityMonitor.swift

@@ -618,7 +618,7 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         let content = UNMutableNotificationContent()
         content.sound = .default
         content.title = "Best Value \(fuel.displayName) nearby: \(brand)"
-        content.body = "\(station.name) could leave you \(result.headline()). Tap for directions."
+        content.body = "\(station.name): \(result.headline()). Tap for directions."
         addAlertRequest(content: content, station: station)
         logAlert(.fired, "Best Value alert scheduled — \(brand) · \(result.headline())")
         lastAlert = "Best Value · \(brand) · \(result.headline())"
@@ -923,7 +923,7 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         content.sound = .default
         let brand = candidate.station.brand.isEmpty ? candidate.station.name : candidate.station.brand
         content.title = "Best Value \(fuel.displayName) nearby: \(brand)"
-        content.body = "\(candidate.station.name) could leave you \(candidate.result.headline()). Tap for directions."
+        content.body = "\(candidate.station.name): \(candidate.result.headline()). Tap for directions."
         let baseResult = "Best Value · \(brand) · \(candidate.result.headline()) · radius \(unit.format(radiusKM))"
         lastTestResult = baseResult
         logAlert(.fired, "Best Value test alert scheduled — \(brand) · \(candidate.result.headline())")

FuelBoard/StationDetailView.swift

@@ -33,6 +33,9 @@ struct StationDetailView: View {
     /// the caption fades out on its own.
     @State private var showingCopied = false
     @State private var copyResetTask: Task<Void, Never>?
+    /// The "How Best Value works" sheet, opened from the row at the bottom of the
+    /// Best Value card (the Key sheet opens the same shared view).
+    @State private var showBestValueInfo = false
 
     private let reportIssueURL = URL(string: "https://www.gov.uk/guidance/report-an-error-in-fuel-prices-or-forecourt-details")!
 
@@ -118,12 +121,92 @@ struct StationDetailView: View {
                         .accessibilityHint(String(localized: "Opens Apple Maps with driving directions",
                                                   comment: "Accessibility hint for the tappable map preview in the More info sheet"))
                     }
-                    .listRowInsets(EdgeInsets())
-                }
-                // The two station actions live in their OWN section, so they
-                // render as a separate card BELOW the map instead of sharing the
-                // map's card and looking attached to the map tap target.
-                Section {
+                    // No hairline between the map and the Address row: the map
+                    // already reads as its own block, so the line only added
+                    // clutter. The Address/Distance and Distance/actions lines
+                    // stay.
+                    .listRowSeparator(.hidden, edges: .bottom)
+                    // The map, the address, the distance, any notices and the two
+                    // station actions all share ONE card (the list's own row
+                    // hairlines separate them), so no hand-rolled Divider here —
+                    // adding one would double the line.
+                    //
+                    // The station name is NOT repeated: the sheet header already
+                    // shows it.
+                    //
+                    // The whole row is a copy control: it puts JUST the postcode
+                    // on the clipboard so it can be pasted into another
+                    // navigation app. The copy glyph sits ON the address line,
+                    // and the "Copied" confirmation is a capsule that floats OVER
+                    // THIS ROW — it confirms the address, so it must not hover
+                    // over the map. It is an overlay, so nothing moves while it is
+                    // up, and `allowsHitTesting(false)` keeps the row tappable
+                    // through the capsule.
+                    Button {
+                        copyPostcode()
+                    } label: {
+                        HStack(alignment: .firstTextBaseline, spacing: 8) {
+                            Text("Address")
+                            Spacer(minLength: 8)
+                            Text(addressLine)
+                                .multilineTextAlignment(.trailing)
+                                .foregroundStyle(.secondary)
+                            // Stays the plain copy glyph — the capsule carries
+                            // the confirmation, so swapping this to a checkmark
+                            // would say it twice.
+                            Image(systemName: "doc.on.doc")
+                                .font(.footnote.weight(.semibold))
+                                .foregroundStyle(.secondary)
+                        }
+                    }
+                    .buttonStyle(.plain)
+                    .disabled(postcodeForCopying.isEmpty)
+                    .accessibilityLabel(String(localized: "Address: \(addressLine)",
+                                               comment: "Accessibility label for the address row in the More info sheet"))
+                    .accessibilityHint(postcodeForCopying.isEmpty
+                        ? ""
+                        : String(localized: "Copies the postcode",
+                                 comment: "Accessibility hint for the address row in the More info sheet"))
+                    // Horizontally centred over the address line itself.
+                    .overlay(alignment: .center) {
+                        if showingCopied {
+                            HStack(spacing: 4) {
+                                Image(systemName: "checkmark")
+                                Text("Copied")
+                            }
+                            .font(.footnote.weight(.semibold))
+                            .foregroundStyle(.green)
+                            .padding(.horizontal, 10)
+                            .padding(.vertical, 6)
+                            // A material base keeps it legible where the tint
+                            // alone (the notice-capsule recipe) washes out.
+                            .background {
+                                Capsule()
+                                    .fill(.regularMaterial)
+                                    .overlay(Capsule().fill(Color.green.opacity(0.22)))
+                            }
+                            .allowsHitTesting(false)
+                            .accessibilityHidden(true)
+                            .transition(.opacity)
+                        }
+                    }
+                    if let location {
+                        let distance = FuelStore.stationDistanceDisplay(
+                            station: station, userLat: location.lat, userLng: location.lng
+                        )
+                        LabeledContent("Distance", value: distance.labelWithReturn(unit: distanceUnit))
+                            .monospacedDigit()
+                    }
+                    ForEach(StationNotice.notices(for: station)) { notice in
+                        Label(notice.title, systemImage: notice.symbolName)
+                            .font(.subheadline.weight(.semibold))
+                            .foregroundStyle(.orange)
+                            .padding(.horizontal, 10)
+                            .padding(.vertical, 6)
+                            .background(Capsule().fill(Color.orange.opacity(0.14)))
+                    }
+                    // The two station actions are the LAST row of the card: half
+                    // the width each, on one line, below Address and Distance.
                     HStack(spacing: 12) {
                         Button {
                             openDirections()
@@ -145,65 +228,6 @@ struct StationDetailView: View {
                     // button its own hit area — the half-row frame above.
                     .buttonStyle(.borderless)
                 }
-                Section {
-                    // The station name is NOT repeated here: the sheet header
-                    // (navigationTitle, inline) already shows it, and the address
-                    // line below has any leading repeat of the name stripped, so
-                    // a "Name" row only said it twice.
-                    //
-                    // The whole row is a copy control: it puts JUST the postcode
-                    // on the clipboard so it can be pasted into another
-                    // navigation app. The copy glyph and its "Copied"
-                    // confirmation share a SECOND line that is always there, so
-                    // the confirmation can never reflow the address above it.
-                    Button {
-                        copyPostcode()
-                    } label: {
-                        VStack(alignment: .leading, spacing: 6) {
-                            HStack(alignment: .firstTextBaseline, spacing: 8) {
-                                Text("Address")
-                                Spacer(minLength: 8)
-                                Text(addressLine)
-                                    .multilineTextAlignment(.trailing)
-                                    .foregroundStyle(.secondary)
-                            }
-                            HStack(spacing: 4) {
-                                Spacer(minLength: 0)
-                                if showingCopied {
-                                    Text("Copied")
-                                        .foregroundStyle(.green)
-                                        .transition(.opacity)
-                                }
-                                Image(systemName: showingCopied ? "checkmark" : "doc.on.doc")
-                                    .foregroundStyle(showingCopied ? Color.green : Color.secondary)
-                            }
-                            .font(.footnote.weight(.semibold))
-                        }
-                    }
-                    .buttonStyle(.plain)
-                    .disabled(postcodeForCopying.isEmpty)
-                    .accessibilityLabel(String(localized: "Address: \(addressLine)",
-                                               comment: "Accessibility label for the address row in the More info sheet"))
-                    .accessibilityHint(postcodeForCopying.isEmpty
-                        ? ""
-                        : String(localized: "Copies the postcode",
-                                 comment: "Accessibility hint for the address row in the More info sheet"))
-                    if let location {
-                        let distance = FuelStore.stationDistanceDisplay(
-                            station: station, userLat: location.lat, userLng: location.lng
-                        )
-                        LabeledContent("Distance", value: distance.labelWithReturn(unit: distanceUnit))
-                            .monospacedDigit()
-                    }
-                    ForEach(StationNotice.notices(for: station)) { notice in
-                        Label(notice.title, systemImage: notice.symbolName)
-                            .font(.subheadline.weight(.semibold))
-                            .foregroundStyle(.orange)
-                            .padding(.horizontal, 10)
-                            .padding(.vertical, 6)
-                            .background(Capsule().fill(Color.orange.opacity(0.14)))
-                    }
-                }
                 // Only shown while Fuel Savings backs it. `fuelSavings` is nil
                 // when the feature is off — `savingsLookup` returns nil when it
                 // is not configured — so the heading cannot be left dangling
@@ -219,6 +243,13 @@ struct StationDetailView: View {
                     } header: {
                         Text("Best Value")
                     }
+
+                    // The explanation lives in its own sheet now, opened from this
+                    // row. The row sits INSIDE the `if let` so it carries the same
+                    // gate as the card above: no card, no row.
+                    Section {
+                        HowBestValueRow { showBestValueInfo = true }
+                    }
                 }
 
 #if DEBUG
@@ -244,6 +275,8 @@ struct StationDetailView: View {
             } message: {
                 Text("Are you sure you want to report an error in fuel prices or forecourt details? This will open in your chosen browser.")
             }
+            // Nested sheet: the More info sheet stays put behind the explanation.
+            .sheet(isPresented: $showBestValueInfo) { HowBestValueSheet() }
             .toolbar {
                 ToolbarItem(placement: .cancellationAction) {
                     Button("Done") { dismiss() }

FuelBoard/StationsView.swift

@@ -35,6 +35,11 @@ struct StationsView: View {
     /// info button in the header — keeps the list focused on stations.
     @State private var showKey = false
 
+    /// The "How Best Value works" sheet. The explanation used to be a section in
+    /// the Key sheet; it is its own sheet now, opened from a row at the bottom of
+    /// the Key sheet and from the More info sheet.
+    @State private var showBestValueInfo = false
+
     /// Station picked for the More-info sheet (tap action = showDetails).
     @State private var detailStation: FuelStation?
 
@@ -218,6 +223,18 @@ struct StationsView: View {
                 if ProcessInfo.processInfo.arguments.contains("-showKeySheet") {
                     showKey = true
                 }
+                // QA hook: `-showBestValueInfo` opens the Key sheet and then the
+                // explanation on top of it, so the copy can be screenshotted
+                // without a tap. It has to chain: the explanation's `.sheet` lives
+                // inside the Key sheet's content, so it cannot present while the
+                // Key sheet is shut.
+                if BestValueInfoQA.isRequested {
+                    showKey = true
+                    Task { @MainActor in
+                        try? await Task.sleep(nanoseconds: 1_200_000_000)
+                        showBestValueInfo = true
+                    }
+                }
 #endif
             }
             .task(id: stations.count) {
@@ -249,7 +266,7 @@ struct StationsView: View {
                             VStack(spacing: 10) {
                                 keyCard(
                                     icon: "checkmark.circle.fill", color: .green,
-                                    title: "Best value", blurb: "Within 1.5p of the cheapest",
+                                    title: "Good price", blurb: "Within 1.5p of the cheapest",
                                     threshold: "≤1.5p")
                                 keyCard(
                                     icon: "equal.circle.fill", color: .orange,
@@ -298,6 +315,14 @@ struct StationsView: View {
                             .listRowBackground(Color.clear)
                             .listRowInsets(EdgeInsets(top: 4, leading: 16, bottom: 4, trailing: 16))
                         }
+
+                        // The explanation is its own sheet now, opened from this
+                        // row. Deliberately NOT gated by Best Value: the Key sheet
+                        // is the legend and is reachable whatever the feature is
+                        // doing.
+                        Section {
+                            HowBestValueRow { showBestValueInfo = true }
+                        }
                     }
                     .navigationTitle("Key")
                     .navigationBarTitleDisplayMode(.inline)
@@ -306,6 +331,8 @@ struct StationsView: View {
                             Button("Done") { showKey = false }
                         }
                     }
+                    // Nested sheet: the Key sheet stays behind the explanation.
+                    .sheet(isPresented: $showBestValueInfo) { HowBestValueSheet() }
                 }
                 .presentationDetents([.fraction(0.6)])
                 .presentationBackground(Color(UIColor.systemGroupedBackground))

FuelBoard/en.lproj/Localizable.strings

@@ -23,7 +23,7 @@
 "Show %lld more (%lld remaining)" = "Show %lld more (%lld remaining)";
 "All %lld stations shown" = "All %lld stations shown";
 "Key" = "Key";
-"Best value" = "Best value";
+"Good price" = "Good price";
 "Within 1.5p of the cheapest" = "Within 1.5p of the cheapest";
 "Okay" = "Okay";
 "Within 4p of the cheapest" = "Within 4p of the cheapest";
@@ -35,6 +35,10 @@
 "Add to favourites" = "Add to favourites";
 "Remove from favourites" = "Remove from favourites";
 "Colours match each station's rating on the list" = "Colours match each station's rating on the list";
+"How Best Value works" = "How Best Value works";
+"Best Value compares each station with the closest one. It works out what you could save at the pump, then takes off the fuel used for the extra return journey." = "Best Value compares each station with the closest one. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.";
+"Your fill-up amount and MPG are used to show whether you could save overall." = "Your fill-up amount and MPG are used to show whether you could save overall.";
+"Results are marked “Approx.” until road distances are available." = "Results are marked “Approx.” until road distances are available.";
 
 /* Favourites tab */
 "No favourites yet" = "No favourites yet";
@@ -199,6 +203,7 @@
 "Cheapest station" = "Cheapest station";
 "Alert type" = "Alert type";
 "Best Value" = "Best Value";
+"Total" = "Total";
 "Minimum saving" = "Minimum saving";
 "Set up Best Value first so FuelBoard can account for your fill size, MPG and travel cost." = "Set up Best Value first so FuelBoard can account for your fill size, MPG and travel cost.";
 "Set up Best Value first so Best Value alerts can account for your fill size, MPG and travel cost." = "Set up Best Value first so Best Value alerts can account for your fill size, MPG and travel cost.";

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsCalculationAuditTests.swift

@@ -82,18 +82,84 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
         XCTAssertEqual(FuelSavingsFormat.distance(1, unit: .miles), DistanceUnit.miles.format(1.609344))
     }
 
-    // MARK: - 2/4. The three stations from the brief
+    // MARK: - 2. Candidate-priced fill-up
+
+    func testPoundsFillUsesCandidatePriceForLitresAndGrossSaving() throws {
+        let result = try FuelSavingsCalculator.calculate(
+            mpg: 30,
+            amount: FuelSavingsFillAmount(value: 60, unit: .pounds),
+            baselinePricePencePerLitre: 171.9,
+            candidatePricePencePerLitre: 169.9,
+            baselineJourneyMiles: 0.879,
+            candidateJourneyMiles: 8.306
+        )
+
+        XCTAssertEqual(result.fillLitres, 60 / 1.699, accuracy: 1e-9)
+        XCTAssertEqual(result.grossSavingGBP, (60 / 1.699) * 0.02, accuracy: 1e-9)
+        XCTAssertEqual(FuelSavingsFormat.money(result.grossSavingGBP), "£0.71")
+        XCTAssertEqual(result.extraFuelLitres, 1.1254603476666667, accuracy: 1e-9)
+        XCTAssertEqual(result.extraDrivingCostGBP, 1.9121571306856667, accuracy: 1e-9)
+        XCTAssertEqual(result.netSavingGBP, -1.2058593084372853, accuracy: 1e-9)
+        XCTAssertEqual(result.headline(), "Costs £1.21 more")
+    }
+
+    func testPoundsExamplesScaleFromThirtyToOneHundredPounds() throws {
+        for (budget, expectedLitres, expectedGross) in [
+            (30.0, 17.657445556209535, 0.3531489111241907),
+            (60.0, 35.31489111241907, 0.7062978222483814),
+            (100.0, 58.85815185403179, 1.1771630370806357)
+        ] {
+            let result = try FuelSavingsCalculator.calculate(
+                mpg: 30,
+                amount: FuelSavingsFillAmount(value: budget, unit: .pounds),
+                baselinePricePencePerLitre: 171.9,
+                candidatePricePencePerLitre: 169.9,
+                baselineJourneyMiles: 0,
+                candidateJourneyMiles: 0
+            )
+            XCTAssertEqual(result.fillLitres, expectedLitres, accuracy: 1e-8)
+            XCTAssertEqual(result.grossSavingGBP, expectedGross, accuracy: 1e-8)
+        }
+    }
+
+    func testCandidatePriceControlsLitresWhileBaselinePriceOnlyChangesSaving() throws {
+        let amount = FuelSavingsFillAmount(value: 60, unit: .pounds)
+        let first = try FuelSavingsCalculator.calculate(
+            mpg: 30, amount: amount,
+            baselinePricePencePerLitre: 171.9,
+            candidatePricePencePerLitre: 169.9,
+            baselineJourneyMiles: 0, candidateJourneyMiles: 0
+        )
+        let differentBaseline = try FuelSavingsCalculator.calculate(
+            mpg: 30, amount: amount,
+            baselinePricePencePerLitre: 179.9,
+            candidatePricePencePerLitre: 169.9,
+            baselineJourneyMiles: 0, candidateJourneyMiles: 0
+        )
+        let differentCandidate = try FuelSavingsCalculator.calculate(
+            mpg: 30, amount: amount,
+            baselinePricePencePerLitre: 179.9,
+            candidatePricePencePerLitre: 159.9,
+            baselineJourneyMiles: 0, candidateJourneyMiles: 0
+        )
+
+        XCTAssertEqual(first.fillLitres, differentBaseline.fillLitres, accuracy: 1e-12)
+        XCTAssertNotEqual(first.grossSavingGBP, differentBaseline.grossSavingGBP)
+        XCTAssertNotEqual(differentBaseline.fillLitres, differentCandidate.fillLitres)
+    }
+
+    // MARK: - 3/5. The three stations from the brief
 
     func testBriefCaseTescoBradfordIsMarginallyWorseOff() throws {
         let result = try briefResult(candidatePrice: 170.9, extraMiles: 2.0)
 
-        XCTAssertEqual(result.fillLitres, 34.904014, accuracy: 1e-6)
-        XCTAssertEqual(result.grossSavingGBP, 0.349040, accuracy: 1e-6, "1p/litre cheaper on a 34.90 L fill")
+        XCTAssertEqual(result.fillLitres, 35.108250, accuracy: 1e-6)
+        XCTAssertEqual(result.grossSavingGBP, 0.351083, accuracy: 1e-6, "£60 buys 35.11 L at the candidate price")
         XCTAssertEqual(result.extraDistanceMiles, 2.0, accuracy: 1e-9)
         XCTAssertEqual(result.extraDrivingCostGBP, 0.517951, accuracy: 1e-6)
-        XCTAssertEqual(result.netSavingGBP, -0.168911, accuracy: 1e-6)
+        XCTAssertEqual(result.netSavingGBP, -0.166869, accuracy: 1e-6)
         XCTAssertEqual(result.status, .notWorthTheDrive)
-        XCTAssertEqual(result.headline(), "£0.17 worse off after travel")
+        XCTAssertEqual(result.headline(), "Costs £0.17 more")
     }
 
     /// The brief recorded "£0.18 worse off" for this row against a displayed
@@ -104,9 +170,9 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
     /// is entirely the rounding of the distance, not of the money.
     func testTheReportedEighteenPenceIsRoundedExtraDistanceNotARoundingBug() throws {
         XCTAssertEqual(try briefResult(candidatePrice: 170.9, extraMiles: 2.00).headline(),
-                       "£0.17 worse off after travel")
+                       "Costs £0.17 more")
         XCTAssertEqual(try briefResult(candidatePrice: 170.9, extraMiles: 2.05).headline(),
-                       "£0.18 worse off after travel")
+                       "Costs £0.18 more")
 
         // 2.05 mi shows as "2.0 mi" at one decimal place — the same caption.
         XCTAssertEqual(FuelSavingsFormat.extraTravelLabel(extraDistanceMiles: 2.05, unit: .miles),
@@ -118,13 +184,13 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
     func testBriefCaseMFGHareAndHoundsIsWorseOff() throws {
         let result = try briefResult(candidatePrice: 172.9, extraMiles: 2.8)
 
-        XCTAssertEqual(result.fillLitres, 34.904014, accuracy: 1e-6)
-        XCTAssertEqual(result.grossSavingGBP, -0.349040, accuracy: 1e-6, "1p/litre dearer costs 34.90 L × 1p")
+        XCTAssertEqual(result.fillLitres, 34.702140, accuracy: 1e-6)
+        XCTAssertEqual(result.grossSavingGBP, -0.347021, accuracy: 1e-6, "£60 buys 34.70 L at the candidate price")
         XCTAssertEqual(result.extraDistanceMiles, 2.8, accuracy: 1e-9)
         XCTAssertEqual(result.extraDrivingCostGBP, 0.733618, accuracy: 1e-6)
-        XCTAssertEqual(result.netSavingGBP, -1.082658, accuracy: 1e-6)
+        XCTAssertEqual(result.netSavingGBP, -1.080639, accuracy: 1e-6)
         XCTAssertEqual(result.status, .notWorthTheDrive)
-        XCTAssertEqual(result.headline(), "£1.08 worse off after travel")
+        XCTAssertEqual(result.headline(), "Costs £1.08 more")
     }
 
     func testBriefCasePGGNapoleonSamePumpPriceIsTravelCostOnly() throws {
@@ -135,7 +201,7 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
         XCTAssertEqual(result.netSavingGBP, -0.729375, accuracy: 1e-6,
                        "with no price difference the whole result is the extra driving")
         XCTAssertEqual(result.status, .notWorthTheDrive)
-        XCTAssertEqual(result.headline(), "£0.73 worse off after travel")
+        XCTAssertEqual(result.headline(), "Costs £0.73 more")
     }
 
     // MARK: - 5/6. Classification, tolerance and rounding
@@ -150,15 +216,15 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
     }
 
     func testSmallSavingInsideTheToleranceIsBreakEvenNotPennies() throws {
-        // 171.9p vs 170.9p over a 1.23 mi delta: net +£0.0305.
+        // 171.9p vs 170.9p over a 1.23 mi delta: the small saving remains within tolerance.
         let result = try briefResult(candidatePrice: 170.9, extraMiles: 1.23)
-        XCTAssertEqual(result.netSavingGBP, 0.030500, accuracy: 1e-6)
+        XCTAssertEqual(result.netSavingGBP, 0.03254252417853143, accuracy: 1e-9)
         XCTAssertEqual(result.status, .breakEven, "±5p reads as break even rather than a meaningless £0.03")
         XCTAssertEqual(result.headline(), "Break even")
     }
 
     func testBreakEvenToleranceDecidesTheClassificationAtTheBoundary() throws {
-        let net = -0.168911
+        let net = -0.166869
         // A tolerance wider than the loss → break even; narrower → worse off.
         let lenient = try FuelSavingsCalculator.calculate(
             mpg: 30,
@@ -187,7 +253,7 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
     func testMoneyIsRoundedOnlyForDisplay() throws {
         // Full precision retained internally…
         let result = try briefResult(candidatePrice: 172.9, extraMiles: 2.8)
-        XCTAssertEqual(result.netSavingGBP, -1.082658, accuracy: 1e-6)
+        XCTAssertEqual(result.netSavingGBP, -1.080639, accuracy: 1e-6)
         XCTAssertNotEqual(result.netSavingGBP, -1.08, "the stored value must not be pre-rounded")
         // …and rounded to two places only when shown.
         XCTAssertEqual(FuelSavingsFormat.money(1.082658), "£1.08")
@@ -237,12 +303,12 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             baselineJourneyMiles: 2.0,
             candidateJourneyMiles: 7.0
         )
-        XCTAssertEqual(result.fillLitres, 27.416038, accuracy: 1e-6)
-        XCTAssertEqual(result.grossSavingGBP, 1.919123, accuracy: 1e-6)
+        XCTAssertEqual(result.fillLitres, 28.797696, accuracy: 1e-6)
+        XCTAssertEqual(result.grossSavingGBP, 2.015839, accuracy: 1e-6)
         XCTAssertEqual(result.extraDrivingCostGBP, 0.701613, accuracy: 1e-6)
-        XCTAssertEqual(result.netSavingGBP, 1.217509, accuracy: 1e-6)
+        XCTAssertEqual(result.netSavingGBP, 1.314226, accuracy: 1e-6)
         XCTAssertEqual(result.status, .worthTheDrive)
-        XCTAssertEqual(result.headline(), "£1.22 better off after travel")
+        XCTAssertEqual(result.headline(), "Save £1.31")
     }
 
     func testNearerCandidateIsNeverChargedExtraTravel() throws {
@@ -270,9 +336,9 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             baselineJourneyMiles: 0.8,
             candidateJourneyMiles: 3.6
         )
-        XCTAssertEqual(sixty.grossSavingGBP, -0.349040, accuracy: 1e-6, "MPG must not touch the pump saving")
+        XCTAssertEqual(sixty.grossSavingGBP, -0.347021, accuracy: 1e-6, "MPG must not touch the pump saving")
         XCTAssertEqual(sixty.extraDrivingCostGBP, 0.366809, accuracy: 1e-6)
-        XCTAssertEqual(sixty.netSavingGBP, -0.715849, accuracy: 1e-6)
+        XCTAssertEqual(sixty.netSavingGBP, -0.713830, accuracy: 1e-6)
 
         let fortyFive = try FuelSavingsCalculator.calculate(
             mpg: 45,
@@ -283,7 +349,7 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             candidateJourneyMiles: 3.6
         )
         XCTAssertEqual(fortyFive.extraDrivingCostGBP, 0.489078, accuracy: 1e-6)
-        XCTAssertEqual(fortyFive.netSavingGBP, -0.838119, accuracy: 1e-6)
+        XCTAssertEqual(fortyFive.netSavingGBP, -0.836100, accuracy: 1e-6)
     }
 
     func testDifferentFillBudgetsScaleOnlyThePumpSaving() throws {
@@ -295,10 +361,10 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             baselineJourneyMiles: 0.8,
             candidateJourneyMiles: 3.6
         )
-        XCTAssertEqual(forty.fillLitres, 23.269343, accuracy: 1e-6, "£40 buys 23.27 L at 171.9p")
-        XCTAssertEqual(forty.grossSavingGBP, -0.232693, accuracy: 1e-6)
+        XCTAssertEqual(forty.fillLitres, 23.134760, accuracy: 1e-6, "£40 buys 23.13 L at the candidate price")
+        XCTAssertEqual(forty.grossSavingGBP, -0.231348, accuracy: 1e-6)
         XCTAssertEqual(forty.extraDrivingCostGBP, 0.733618, accuracy: 1e-6, "the journey cost is budget-independent")
-        XCTAssertEqual(forty.netSavingGBP, -0.966311, accuracy: 1e-6)
+        XCTAssertEqual(forty.netSavingGBP, -0.964965, accuracy: 1e-6)
 
         let ninety = try FuelSavingsCalculator.calculate(
             mpg: 30,
@@ -308,9 +374,9 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             baselineJourneyMiles: 0.8,
             candidateJourneyMiles: 3.6
         )
-        XCTAssertEqual(ninety.fillLitres, 52.356021, accuracy: 1e-6)
-        XCTAssertEqual(ninety.grossSavingGBP, -0.523560, accuracy: 1e-6)
-        XCTAssertEqual(ninety.netSavingGBP, -1.257178, accuracy: 1e-6)
+        XCTAssertEqual(ninety.fillLitres, 52.053210, accuracy: 1e-6)
+        XCTAssertEqual(ninety.grossSavingGBP, -0.520532, accuracy: 1e-6)
+        XCTAssertEqual(ninety.netSavingGBP, -1.254150, accuracy: 1e-6)
     }
 
     func testFillUpInLitresIsFixedRegardlessOfPrice() throws {
@@ -370,7 +436,7 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
             journey: journey(2.8),
             baselineJourney: journey(0.8)
         )
-        XCTAssertEqual(both?.result?.netSavingGBP ?? 0, -0.168911, accuracy: 1e-6)
+        XCTAssertEqual(both?.result?.netSavingGBP ?? 0, -0.166869, accuracy: 1e-6)
     }
 
     func testMixedRouteBasisIsReportedAsAnEstimate() {
@@ -393,7 +459,7 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
         XCTAssertEqual(mixed?.result?.distanceBasis, .straightLine,
                        "one unmeasured leg makes the whole estimate an estimate")
         XCTAssertEqual(mixed?.result?.journeyConfidence, .estimated)
-        XCTAssertEqual(mixed?.result?.headline(compact: true), "Approx. £0.17 worse off after travel")
+        XCTAssertEqual(mixed?.result?.headline(compact: true), "Approx. Costs £0.17 more")
     }
 
     func testBaselineStationIsLabelledClosestStationNotBreakEven() {

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsEstimateTests.swift

@@ -284,9 +284,9 @@ final class FuelSavingsEstimateTests: XCTestCase {
     /// (171.9p, 0.4 mi), candidate Shell Whetley Hill (167.9p, 3.8 mi).
     ///
     /// Whetley Hill is 4p/litre cheaper, yet it is WORSE OFF — and that is
-    /// correct, not a calculation bug: £60 buys 34.90 L at 171.9p, so the
-    /// cheaper pump saves 34.90 × 4p = £1.40, while the extra 6.8 return miles
-    /// burn 6.8/30 × 4.54609 = 1.03 L at 167.9p = £1.73. Net ≈ −£0.33.
+    /// correct, not a calculation bug: £60 buys 35.74 L at 167.9p, so the
+    /// cheaper pump saves 35.74 × 4p = £1.43, while the extra 6.8 return miles
+    /// burn 6.8/30 × 4.54609 = 1.03 L at 167.9p = £1.73. Net ≈ −£0.30.
     func testCheaperStationFurtherAwayIsWorseOffAfterTravel() {
         let baseline = station("rontec-shelf", price: 171.9)
         let candidate = station("shell-whetley-hill", price: 167.9)
@@ -304,16 +304,16 @@ final class FuelSavingsEstimateTests: XCTestCase {
         guard let result = availability?.result else {
             return XCTFail("expected an estimate, got \(String(describing: availability))")
         }
-        XCTAssertEqual(result.fillLitres, 34.904, accuracy: 0.005,
-                       "the £60 budget buys litres at the BASELINE pump price")
-        XCTAssertEqual(result.grossSavingGBP, 1.396, accuracy: 0.005)
+        XCTAssertEqual(result.fillLitres, 35.736, accuracy: 0.005,
+                       "the £60 budget buys litres at the candidate pump price")
+        XCTAssertEqual(result.grossSavingGBP, 1.429, accuracy: 0.005)
         XCTAssertEqual(result.extraDistanceMiles, 6.8, accuracy: 0.001,
                        "only the return-trip DELTA counts — the shared journey is never counted twice")
         XCTAssertEqual(result.extraDrivingCostGBP, 1.730, accuracy: 0.005)
-        XCTAssertEqual(result.netSavingGBP, -0.334, accuracy: 0.005)
+        XCTAssertEqual(result.netSavingGBP, -0.301, accuracy: 0.005)
         XCTAssertEqual(result.status, .notWorthTheDrive)
         XCTAssertEqual(result.journeyConfidence, .estimated)
-        XCTAssertEqual(result.headline(compact: true), "Approx. £0.33 worse off after travel")
+        XCTAssertEqual(result.headline(compact: true), "Approx. Costs £0.30 more")
     }
 
     func testMeasuredRoadRoutesDropTheEstimateMarker() {
@@ -332,8 +332,8 @@ final class FuelSavingsEstimateTests: XCTestCase {
 
         XCTAssertEqual(availability?.result?.distanceBasis, .roadRoute)
         XCTAssertEqual(availability?.result?.journeyConfidence, .confirmed)
-        XCTAssertEqual(availability?.result?.headline(compact: true), "£0.33 worse off after travel")
-        XCTAssertEqual(availability?.result?.headline(), "£0.33 worse off after travel")
+        XCTAssertEqual(availability?.result?.headline(compact: true), "Costs £0.30 more")
+        XCTAssertEqual(availability?.result?.headline(), "Costs £0.30 more")
     }
 
     func testBaselineRowIsLabelledClosestStation() {
@@ -375,10 +375,10 @@ final class FuelSavingsEstimateTests: XCTestCase {
             return XCTFail("expected an estimate, got \(String(describing: availability))")
         }
         XCTAssertEqual(result.status, .worthTheDrive)
-        // £60 buys 37.5 L at 160p; 10p/litre cheaper saves £3.75; 0.4 extra
-        // return miles cost ~£0.09. Net ≈ £3.66.
-        XCTAssertEqual(result.netSavingGBP, 3.659, accuracy: 0.005)
-        XCTAssertEqual(result.headline(compact: true), "Approx. £3.66 better off after travel")
+        // £60 buys 40 L at 150p; 10p/litre cheaper saves £4.00; 0.4 extra
+        // return miles cost ~£0.09. Net ≈ £3.91.
+        XCTAssertEqual(result.netSavingGBP, 3.909, accuracy: 0.005)
+        XCTAssertEqual(result.headline(compact: true), "Approx. Save £3.91")
     }
 
     func testNeutralResultHeadlineIsBreakEven() {
@@ -413,7 +413,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
                                        netSavingGBP: 0.52,
                                        status: .worthTheDrive,
                                        distanceBasis: .roadRoute)
-        XCTAssertEqual(saving.headline(), "£0.52 better off after travel")
+        XCTAssertEqual(saving.headline(), "Save £0.52")
 
         let loss = FuelSavingsResult(fillLitres: 34.9,
                                      grossSavingGBP: 1.40,
@@ -422,7 +422,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
                                      netSavingGBP: -0.31,
                                      status: .notWorthTheDrive,
                                      distanceBasis: .roadRoute)
-        XCTAssertEqual(loss.headline(), "£0.31 worse off after travel")
+        XCTAssertEqual(loss.headline(), "Costs £0.31 more")
     }
 
     /// The sort must order by NET saving, not pump price: a pricier station can
@@ -497,7 +497,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             status: .worthTheDrive,
             distanceBasis: .straightLine
         )
-        XCTAssertEqual(saving.capsuleHeadline(), "Approx. £1.00 better off after travel")
+        XCTAssertEqual(saving.capsuleHeadline(), "Approx. Save £1.00")
 
         let worse = FuelSavingsResult(
             fillLitres: 30,
@@ -507,7 +507,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             netSavingGBP: -0.31,
             status: .notWorthTheDrive
         )
-        XCTAssertEqual(worse.capsuleHeadline(), "£0.31 worse off after travel")
+        XCTAssertEqual(worse.capsuleHeadline(), "Costs £0.31 more")
     }
 
     // MARK: - Estimate wording (Approx. labels)
@@ -524,7 +524,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             status: .worthTheDrive,
             distanceBasis: .straightLine
         )
-        XCTAssertEqual(saving.headline(), "Approx. £1.25 better off after travel")
+        XCTAssertEqual(saving.headline(), "Approx. Save £1.25")
 
         let loss = FuelSavingsResult(
             fillLitres: 30,
@@ -535,7 +535,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             status: .notWorthTheDrive,
             distanceBasis: .straightLine
         )
-        XCTAssertEqual(loss.headline(), "Approx. £0.77 worse off after travel")
+        XCTAssertEqual(loss.headline(), "Approx. Costs £0.77 more")
     }
 
     /// A confirmed road-route comparison must never carry the marker — in either
@@ -551,7 +551,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             distanceBasis: .roadRoute,
             journeyConfidence: .confirmed
         )
-        XCTAssertEqual(saving.headline(), "£1.25 better off after travel")
+        XCTAssertEqual(saving.headline(), "Save £1.25")
 
         let loss = FuelSavingsResult(
             fillLitres: 30,
@@ -563,7 +563,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             distanceBasis: .roadRoute,
             journeyConfidence: .confirmed
         )
-        XCTAssertEqual(loss.headline(), "£0.77 worse off after travel")
+        XCTAssertEqual(loss.headline(), "Costs £0.77 more")
 
         for headline in [saving.headline(), loss.headline(), saving.capsuleHeadline()] {
             XCTAssertFalse(headline.contains("Approx"), "\"\(headline)\" must not claim to be an estimate")
@@ -597,7 +597,7 @@ final class FuelSavingsEstimateTests: XCTestCase {
             status: .notWorthTheDrive,
             distanceBasis: .straightLine
         )
-        XCTAssertEqual(estimated.spokenHeadline(), "Approximately £0.77 worse off after travel")
+        XCTAssertEqual(estimated.spokenHeadline(), "Approximately Costs £0.77 more")
 
         XCTAssertEqual(
             FuelSavingsFormat.extraTravelSpokenLabel(extraDistanceMiles: 6.4, unit: .miles, confidence: .estimated),

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsTests.swift

@@ -8,15 +8,15 @@ final class FuelSavingsCalculatorTests: XCTestCase {
         XCTAssertEqual(FuelSavingsCalculator.fuelUsedLitres(distanceMiles: 10, mpg: 45), 1.0102, accuracy: 0.0001)
     }
 
-    func testPoundsBudgetConvertsToLitresUsingBaselinePrice() throws {
+    func testPoundsBudgetConvertsToLitresUsingGivenPrice() throws {
         let amount = FuelSavingsFillAmount(value: 40, unit: .pounds)
-        let litres = try FuelSavingsCalculator.fillLitres(amount: amount, baselinePricePencePerLitre: 160)
+        let litres = try FuelSavingsCalculator.fillLitres(amount: amount, pricePencePerLitre: 160)
         XCTAssertEqual(litres, 25.0, accuracy: 0.0001)
     }
 
     func testLitresAmountPassesThrough() throws {
         let amount = FuelSavingsFillAmount(value: 35, unit: .litres)
-        let litres = try FuelSavingsCalculator.fillLitres(amount: amount, baselinePricePencePerLitre: 160)
+        let litres = try FuelSavingsCalculator.fillLitres(amount: amount, pricePencePerLitre: 160)
         XCTAssertEqual(litres, 35, accuracy: 0.0001)
     }
 
@@ -29,11 +29,11 @@ final class FuelSavingsCalculatorTests: XCTestCase {
             baselineJourneyMiles: 2,
             candidateJourneyMiles: 8
         )
-        XCTAssertEqual(result.fillLitres, 25, accuracy: 0.0001)
-        XCTAssertEqual(result.grossSavingGBP, 2.50, accuracy: 0.0001)
+        XCTAssertEqual(result.fillLitres, 26.6667, accuracy: 0.0001)
+        XCTAssertEqual(result.grossSavingGBP, 2.6667, accuracy: 0.0001)
         XCTAssertEqual(result.extraDistanceMiles, 6, accuracy: 0.0001)
         XCTAssertEqual(result.extraDrivingCostGBP, 0.9092, accuracy: 0.0001)
-        XCTAssertEqual(result.netSavingGBP, 1.5908, accuracy: 0.0001)
+        XCTAssertEqual(result.netSavingGBP, 1.7574, accuracy: 0.0001)
         XCTAssertEqual(result.status, .worthTheDrive)
     }
 
@@ -61,7 +61,7 @@ final class FuelSavingsCalculatorTests: XCTestCase {
     }
 
     func testInvalidInputsRejected() {
-        XCTAssertThrowsError(try FuelSavingsCalculator.fillLitres(amount: FuelSavingsFillAmount(value: 40, unit: .pounds), baselinePricePencePerLitre: 0))
+        XCTAssertThrowsError(try FuelSavingsCalculator.fillLitres(amount: FuelSavingsFillAmount(value: 40, unit: .pounds), pricePencePerLitre: 0))
         XCTAssertThrowsError(try FuelSavingsCalculator.calculate(mpg: 0, amount: FuelSavingsFillAmount(value: 40, unit: .pounds), baselinePricePencePerLitre: 160, candidatePricePencePerLitre: 150, baselineJourneyMiles: 1, candidateJourneyMiles: 2))
         XCTAssertThrowsError(try FuelSavingsCalculator.calculate(mpg: 500, amount: FuelSavingsFillAmount(value: 40, unit: .pounds), baselinePricePencePerLitre: 160, candidatePricePencePerLitre: 150, baselineJourneyMiles: 1, candidateJourneyMiles: 2))
     }

Shared/FuelSavings.swift

@@ -82,6 +82,7 @@ enum JourneyConfidence: String, Codable, Equatable {
 struct FuelSavingsResult: Codable, Equatable {
     let fillLitres: Double
     let grossSavingGBP: Double
+    let extraFuelLitres: Double
     let extraDrivingCostGBP: Double
     let extraDistanceMiles: Double
     let netSavingGBP: Double
@@ -91,6 +92,7 @@ struct FuelSavingsResult: Codable, Equatable {
 
     init(fillLitres: Double,
          grossSavingGBP: Double,
+         extraFuelLitres: Double = 0,
          extraDrivingCostGBP: Double,
          extraDistanceMiles: Double,
          netSavingGBP: Double,
@@ -99,6 +101,7 @@ struct FuelSavingsResult: Codable, Equatable {
          journeyConfidence: JourneyConfidence? = nil) {
         self.fillLitres = fillLitres
         self.grossSavingGBP = grossSavingGBP
+        self.extraFuelLitres = extraFuelLitres
         self.extraDrivingCostGBP = extraDrivingCostGBP
         self.extraDistanceMiles = extraDistanceMiles
         self.netSavingGBP = netSavingGBP
@@ -174,6 +177,7 @@ extension FuelSavingsResult {
         FuelSavingsResult(
             fillLitres: fillLitres,
             grossSavingGBP: 0,
+            extraFuelLitres: 0,
             extraDrivingCostGBP: 0,
             extraDistanceMiles: 0,
             netSavingGBP: 0,
@@ -199,9 +203,9 @@ extension FuelSavingsResult {
         case .breakEven:
             return Self.breakEvenHeadline
         case .worthTheDrive:
-            return "\(prefix)\(FuelSavingsFormat.money(netSavingGBP)) better off after travel"
+            return "\(prefix)Save \(FuelSavingsFormat.money(netSavingGBP))"
         case .notWorthTheDrive:
-            return "\(prefix)\(FuelSavingsFormat.money(abs(netSavingGBP))) worse off after travel"
+            return "\(prefix)Costs \(FuelSavingsFormat.money(abs(netSavingGBP))) more"
         }
     }
 
@@ -321,16 +325,16 @@ enum FuelSavingsCalculator {
         distanceMiles / mpg * imperialGallonLitres
     }
 
-    static func fillLitres(amount: FuelSavingsFillAmount, baselinePricePencePerLitre: Double) throws -> Double {
+    static func fillLitres(amount: FuelSavingsFillAmount, pricePencePerLitre: Double) throws -> Double {
         guard amount.isValid else { throw FuelSavingsError.invalidAmount }
         switch amount.unit {
         case .litres:
             return amount.value
         case .pounds:
-            guard baselinePricePencePerLitre.isFinite, baselinePricePencePerLitre > 0 else {
+            guard pricePencePerLitre.isFinite, pricePencePerLitre > 0 else {
                 throw FuelSavingsError.invalidPrice
             }
-            return amount.value / (baselinePricePencePerLitre / 100.0)
+            return amount.value / (pricePencePerLitre / 100.0)
         }
     }
 
@@ -376,7 +380,7 @@ enum FuelSavingsCalculator {
               baselineJourneyMiles >= 0, candidateJourneyMiles >= 0 else {
             throw FuelSavingsError.invalidDistance
         }
-        let fillLitres = try fillLitres(amount: amount, baselinePricePencePerLitre: baselinePricePencePerLitre)
+        let fillLitres = try fillLitres(amount: amount, pricePencePerLitre: candidatePricePencePerLitre)
         let baselinePriceGBP = baselinePricePencePerLitre / 100.0
         let candidatePriceGBP = candidatePricePencePerLitre / 100.0
         let grossSaving = fillLitres * (baselinePriceGBP - candidatePriceGBP)
@@ -395,6 +399,7 @@ enum FuelSavingsCalculator {
         return FuelSavingsResult(
             fillLitres: fillLitres,
             grossSavingGBP: grossSaving,
+            extraFuelLitres: extraFuelLitres,
             extraDrivingCostGBP: extraFuelCost,
             extraDistanceMiles: extraDistance,
             netSavingGBP: netSaving,