The hard part of this sum is never the multiplication - it is that consumption is written four different ways, two of them called mpg with different gallons behind them. This page converts whichever one your car uses, then gives you the litres, the money and the share per passenger.

How to use it

1. 1 Enter the distance and tick round trip if you are coming back.
2. 2 Pick the unit your car reports and type its figure.
3. 3 Add the pump price and the number of people sharing.

Four units, two of them named the same

Two of these count fuel for a fixed distance, so lower is better. The other two count distance for a fixed amount of fuel, so higher is better. Reading one as the other turns an economical car into a thirsty one.

A US gallon is not a UK gallon

The UK gallon holds about 20% more, so the same car is quoted at a higher mpg in Britain than in America without being any more efficient. Comparing a British review with an American one, unit unnoticed, makes one car look far better than it is.

Real consumption is worse than the brochure

Official figures come from a standard test cycle, not from your commute. Cold starts, city traffic, a roof box, air conditioning and motorway speed all push consumption up, often by a fifth or more. If you have refuelled twice, your own numbers from the odometer beat any published figure.

Fuel is only part of what driving costs

Tyres, servicing, insurance and depreciation all accrue with distance, and together they usually exceed the fuel. This page answers the narrow question people actually ask before a trip; for comparing a car against a train, add those other costs by hand.