Should I use the official or real-world consumption figure?

Use what your car actually reports over a few tanks. Official figures come from a test cycle and are usually 10 to 25 per cent optimistic in mixed real driving.

How do I work out cost per mile?

Divide the total trip cost by the distance. The calculator shows cost per kilometre; multiply by 1.609 for cost per mile.

Does this work for an electric car?

The same arithmetic applies if you substitute kWh per 100 km for litres per 100 km and your electricity price per kWh for the pump price.

Fuel Cost Calculator

Quick answer

Trip fuel cost equals distance ÷ 100 × consumption in litres per 100 km × price per litre. At 7 L/100 km and €1.70 a litre, 600 km costs about €71.

What will this trip cost in fuel?

Distance, consumption and pump price — any currency.

Distance (km)

Consumption (L per 100 km)

US mpg → L/100 km: divide 235.2 by the mpg figure.

Fuel price per litre

Trip fuel cost

71.40

Litres used

42.0 L

Cost per km

0.119

Return journey? Double the distance. Real consumption runs 10–25% above the official figure at motorway speed.

Fuel is the only car cost that scales directly with distance, which makes it the number worth checking before a long drive or when comparing driving with the train. Everything else — insurance, depreciation, road tax — you pay whether the car moves or not.

Real-world consumption is reliably worse than the official figure. Manufacturer numbers come from a standardised test cycle; motorway speeds, a roof box, cold weather and a full load all push actual consumption up, often by 10 to 25 per cent.

Speed costs more than people think

Aerodynamic drag rises with the square of speed, and the power needed to overcome it with the cube. Driving at 120 km/h instead of 100 km/h typically raises consumption by around 20 per cent while saving well under ten minutes an hour.

A roof box or roof bars can add 10 to 20 per cent at motorway speed even when empty. Taking them off between trips is one of the few changes that pays for itself immediately.

Converting between consumption units

Litres per 100 km and miles per gallon move in opposite directions, which makes them easy to confuse. To convert US mpg to L/100 km, divide 235.2 by the mpg figure; for UK mpg, divide 282.5. So 30 US mpg is about 7.8 L/100 km.

Because the relationship is a reciprocal, improvements at the thirsty end matter far more. Going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50 mpg.

Frequently asked questions

Should I use the official or real-world consumption figure?: Use what your car actually reports over a few tanks. Official figures come from a test cycle and are usually 10 to 25 per cent optimistic in mixed real driving.
How do I work out cost per mile?: Divide the total trip cost by the distance. The calculator shows cost per kilometre; multiply by 1.609 for cost per mile.
Does this work for an electric car?: The same arithmetic applies if you substitute kWh per 100 km for litres per 100 km and your electricity price per kWh for the pump price.

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Updated 2026-09-20 · all calculators. Results are estimates for guidance, not professional advice.