Fuel Cost Calculator

Enter the distance, the car's fuel economy and the price of fuel, and you get the total cost, how much fuel it burns and the cost per kilometre or mile. Metric uses litres per 100 km and price per litre; US units use miles per gallon and price per gallon. Tick round trip to double the distance.

The two economy scales are not just different units, they run in opposite directions, and one of them is actively misleading. Litres per 100 km is fuel per distance, so it is proportional to what you spend: halve the figure and you halve the cost. Miles per gallon is distance per fuel, a reciprocal, and equal steps on it are not equal savings. Over a thousand miles, improving from 10 to 15 mpg saves 33 gallons. Improving from 30 to 35 mpg — the same five-mpg step — saves 4.8. Seven times less, for what looks on paper like the same upgrade. This is why replacing the thirstiest vehicle in a fleet matters far more than making an efficient one slightly better, and why economists have a name for the effect.

Gallons are also not interchangeable. A US gallon is 3.785 litres and an imperial gallon is 4.546, about twenty percent larger, so a British car quoted at 48 mpg and an American one at 40 mpg are doing roughly the same thing. A figure copied from a UK review into a US calculator will overstate the range by a fifth.

Two things the arithmetic cannot know. The rated economy is measured on a test cycle and real driving rarely matches it — short trips where the engine never warms, cold weather, a roof box, speed above about 100 km/h where drag dominates, and air conditioning all cost several percent each, and they compound. And fuel is not the cost of the journey: tyres, servicing, depreciation and tolls are real and mostly proportional to distance too, which is what makes a fuel-only figure look so favourable next to a train ticket.