Distance and MPG are only the start
A basic journey fuel cost calculator multiplies distance by a fuel-consumption estimate. That is useful, but it does not tell you whether a cheaper fuel stop is worth adding to the trip.
For real driving decisions, the calculation should include the route, the fuel type, the expected fill amount and the extra fuel used by any detour.
The simple journey fuel formula
Estimated litres used = journey distance × litres per 100km ÷ 100.
Estimated fuel cost = estimated litres used × pump price per litre.
If you think in UK MPG, convert MPG to litres per 100km first. UK gallons are larger than US gallons, so use UK MPG values when comparing UK vehicles.
What changes the result?
- Urban versus motorway driving: stop-start traffic and short trips usually use more fuel than steady cruising.
- Your vehicle: tank size, fuel type and real-world MPG change the useful saving.
- Speed and smoothness: harsh braking, acceleration and under-inflated tyres can worsen consumption.
- Detours: a cheaper station can lose its advantage if the extra driving costs too much fuel.
- Live traffic: traffic can add delay and affect fuel use, so it should be shown alongside the saving.
How Route & Fuel applies this
The app is designed to use vehicle MPG and tank details, then compare fuel stops using latest published prices, route position, detour fuel cost and live traffic timing. The goal is not to claim an exact real-world MPG; it is to produce a useful estimate that is closer to the driver’s actual journey.
Best wording for drivers
“Estimated MPG” and “estimated fuel saving” are honest terms. They make clear that weather, driving style, load, tyre pressure and traffic can change the final result.