UX Improvements

backend/src/features/fuel-logs/domain/efficiency-calculation.service.ts

@@ -37,7 +37,7 @@ export class EfficiencyCalculationService {
       return null;
     }
 
-    const value = UnitConversionService.calculateEfficiency(distance, currentLog.fuelUnits, unitSystem);
+    const value = UnitConversionService.calculateEfficiency(distance, currentLog.fuelUnits);
     const labels = UnitConversionService.getUnitLabels(unitSystem);
     return { value, unitSystem, label: labels.efficiencyUnits, calculationMethod: method };
   }

backend/src/features/fuel-logs/domain/fuel-logs.service.ts

@@ -85,7 +85,17 @@ export class FuelLogsService {
     if (cached) return cached;
 
     const rows = await this.repository.findByVehicleIdEnhanced(vehicleId);
-    const response = rows.map(r => this.toEnhancedResponse(r, undefined, unitSystem));
+    const response = rows.map(r => {
+      const efficiency = EfficiencyCalculationService.calculateEfficiency(
+        {
+          tripDistance: r.trip_distance ?? undefined,
+          fuelUnits: r.fuel_units ?? undefined
+        },
+        null, // No previous odometer needed for trip distance calculation
+        unitSystem
+      );
+      return this.toEnhancedResponse(r, efficiency?.value ?? undefined, unitSystem);
+    });
     await cacheService.set(cacheKey, response, this.cacheTTL);
     return response;
   }
@@ -96,7 +106,17 @@ export class FuelLogsService {
     const cached = await cacheService.get<EnhancedFuelLogResponse[]>(cacheKey);
     if (cached) return cached;
     const rows = await this.repository.findByUserIdEnhanced(userId);
-    const response = rows.map(r => this.toEnhancedResponse(r, undefined, unitSystem));
+    const response = rows.map(r => {
+      const efficiency = EfficiencyCalculationService.calculateEfficiency(
+        {
+          tripDistance: r.trip_distance ?? undefined,
+          fuelUnits: r.fuel_units ?? undefined
+        },
+        null, // No previous odometer needed for trip distance calculation
+        unitSystem
+      );
+      return this.toEnhancedResponse(r, efficiency?.value ?? undefined, unitSystem);
+    });
     await cacheService.set(cacheKey, response, this.cacheTTL);
     return response;
   }
@@ -106,7 +126,17 @@ export class FuelLogsService {
     if (!row) throw new Error('Fuel log not found');
     if (row.user_id !== userId) throw new Error('Unauthorized');
     const { unitSystem } = await UserSettingsService.getUserSettings(userId);
-    return this.toEnhancedResponse(row, undefined, unitSystem);
+
+    const efficiency = EfficiencyCalculationService.calculateEfficiency(
+      {
+        tripDistance: row.trip_distance ?? undefined,
+        fuelUnits: row.fuel_units ?? undefined
+      },
+      null, // No previous odometer needed for trip distance calculation
+      unitSystem
+    );
+
+    return this.toEnhancedResponse(row, efficiency?.value ?? undefined, unitSystem);
   }
 
   async updateFuelLog(id: string, data: EnhancedUpdateFuelLogRequest, userId: string): Promise<EnhancedFuelLogResponse> {

backend/src/features/fuel-logs/domain/unit-conversion.service.ts

@@ -3,28 +3,29 @@ import { UnitSystem as CoreUnitSystem } from '../../../shared-minimal/utils/unit
 export type UnitSystem = CoreUnitSystem;
 
 export class UnitConversionService {
-  private static readonly MPG_TO_L100KM = 235.214;
+  private static readonly KM_PER_MILE = 1.60934;
+  private static readonly GALLONS_TO_LITERS = 3.78541;
 
   static getUnitLabels(unitSystem: UnitSystem) {
     return unitSystem === 'metric'
-      ? { fuelUnits: 'liters', distanceUnits: 'kilometers', efficiencyUnits: 'L/100km' }
-      : { fuelUnits: 'gallons', distanceUnits: 'miles', efficiencyUnits: 'mpg' };
+      ? { fuelUnits: 'liters', distanceUnits: 'kilometers', efficiencyUnits: 'km/L' }
+      : { fuelUnits: 'gallons', distanceUnits: 'miles', efficiencyUnits: 'MPG' };
   }
 
-  static calculateEfficiency(distance: number, fuelUnits: number, unitSystem: UnitSystem): number {
+  static calculateEfficiency(distance: number, fuelUnits: number): number {
     if (fuelUnits <= 0 || distance <= 0) return 0;
-    return unitSystem === 'metric'
-      ? (fuelUnits / distance) * 100
-      : distance / fuelUnits;
+    return distance / fuelUnits; // Both km/L and MPG use distance/fuel formula
   }
 
   static convertEfficiency(efficiency: number, from: UnitSystem, to: UnitSystem): number {
     if (from === to) return efficiency;
     if (from === 'imperial' && to === 'metric') {
-      return efficiency > 0 ? this.MPG_TO_L100KM / efficiency : 0;
+      // MPG to km/L: MPG * (gallons_to_liters / km_per_mile)
+      return efficiency * (this.GALLONS_TO_LITERS / this.KM_PER_MILE);
     }
     if (from === 'metric' && to === 'imperial') {
-      return efficiency > 0 ? this.MPG_TO_L100KM / efficiency : 0;
+      // km/L to MPG: km/L * (km_per_mile / gallons_to_liters)
+      return efficiency * (this.KM_PER_MILE / this.GALLONS_TO_LITERS);
     }
     return efficiency;
   }