UK Fuel Efficiency Comparison Calculator

Choosing between two cars often comes down to a gut feeling about which one seems more economical, without actually running the numbers. This calculator compares two vehicles side by side on annual fuel cost, based on your own mileage, each car's MPG, and current fuel prices, so you can see the real pound-for-pound difference rather than guessing from a badge or engine size.

How to use the fuel efficiency comparison calculator

Enter your expected annual mileage, MPG, fuel type and price per litre for Vehicle A, then do the same for Vehicle B. You can use the same annual mileage for both if you are comparing two cars for the same use case, or different figures if the cars would genuinely be driven differently. The calculator shows each vehicle's annual fuel cost, cost per mile, and the total difference between them, both per year and projected over 5 years.

How the calculation works

Fuel used is calculated from your annual mileage divided by MPG, converted from UK gallons into litres, then multiplied by your fuel price per litre. This mirrors the calculation behind our fuel cost calculator, applied twice so you can see two vehicles next to each other directly. Because petrol and diesel are priced differently per litre, and MPG figures often differ meaningfully between fuel types, comparing raw MPG numbers alone can be misleading. This calculator converts everything into a pounds-and-pence annual cost, which is the number that actually affects your budget.

Worked example

Take Vehicle A, a petrol car doing 45 MPG at £1.45 a litre, against Vehicle B, a diesel car doing 60 MPG at £1.52 a litre, both driven 10,000 miles a year.

Vehicle A uses more fuel overall due to its lower MPG, but pays a lower price per litre. Vehicle B uses noticeably less fuel thanks to its higher MPG, despite diesel costing more per litre. Once both are converted into an annual cost, the calculator shows exactly which vehicle comes out cheaper and by how much, and projects that difference over 5 years, since a gap that looks small annually can add up to a meaningful sum over a typical ownership period.

Try adjusting the annual mileage upward in the calculator above. You will see the annual and 5-year cost difference between the two vehicles grow larger, since higher mileage magnifies any efficiency gap between two cars. This is exactly why high-mileage drivers tend to weigh MPG more heavily when choosing a car than low-mileage drivers do.

What affects real-world fuel efficiency

MPG figures are heavily influenced by driving style and conditions, not just the car itself. Motorway driving at a steady speed typically achieves closer to a car's official figures than stop-start urban driving, which can reduce real-world MPG noticeably, particularly for larger or heavier vehicles. Carrying extra weight, using air conditioning heavily, and short journeys that never let the engine reach optimal temperature can all reduce the MPG you actually achieve compared with the headline figure.

Hybrid vehicles are a particular case worth understanding. They often perform best in exactly the conditions that hurt petrol and diesel cars most: slow, stop-start urban driving, where the electric motor can do more of the work and regenerative braking recovers energy that would otherwise be wasted. On a long motorway journey, a hybrid's advantage over an efficient diesel often narrows considerably, so the type of driving you mostly do matters as much as the vehicle type when comparing efficiency.

Common mistakes to avoid

A common mistake is comparing manufacturer MPG figures directly without adjusting for real-world driving. Official test figures are measured under controlled conditions and tend to be optimistic compared with typical UK driving, particularly for shorter journeys and urban driving with lots of stopping and starting. Where possible, use a realistic real-world MPG figure, either from your own experience or from independent real-world test data, rather than the headline manufacturer number.

It is also easy to focus purely on fuel cost and forget the other running costs that can outweigh a small MPG advantage. A car with slightly worse fuel economy but noticeably lower insurance, servicing or road tax costs can still work out cheaper overall. Fuel is usually the single biggest running cost for higher-mileage drivers, but it is rarely the only one worth comparing.

Finally, remember that fuel prices themselves change regularly. Running this comparison with today's prices gives a snapshot, but it is worth revisiting if fuel prices move significantly, or before a major purchase decision, rather than relying on a comparison done many months earlier.

Tyre pressure, regular servicing, and even removing a roof box or unused roof bars can also make a small but genuine difference to real-world MPG. None of these factors show up in a manufacturer specification sheet, but they are worth keeping in mind once you have chosen a car, since they can help you get closer to its official efficiency figures in everyday driving.

Related calculators

For a single vehicle's running cost on a specific journey, our fuel cost calculator and MPG calculator can help. Check your annual road tax (VED) cost by fuel type and CO2 emissions to add to the comparison, and if you drive in London, our London driving charges guide covers the ULEZ and Congestion Charge costs that can also differ significantly between vehicles.