MPG, litres per 100 km and fuel cost per mile explained

Fuel is one of the largest running costs of a car, and the figures used to describe it are a muddle. The brochure says miles per gallon, the pump says pence per litre, some tools use litres per 100 kilometres, and an imperial gallon is not the gallon used in America. This guide shows how to work out your real fuel economy from a tank of fuel, how to convert between the different measures, how to turn the result into a cost per mile and the cost of a journey, and why the number on the brochure is rarely what you get. All the examples use UK gallons and an example fuel price of 140 pence per litre, which you can replace with the price you pay.

By Valtrinity. Updated 18 October 2026.

What mpg means

Miles per gallon, or mpg, is how many miles a car travels on one gallon of fuel. In the UK, the gallon is the imperial gallon of 4.54609 litres. A higher number is better, because the car goes further for the same fuel.

The system has a quirk: the gains do not add up evenly. Improving from 20 to 30 mpg saves much more fuel over 1,000 miles than improving from 40 to 50 mpg. At 20 mpg, 1,000 miles takes 50 gallons, and at 30 mpg it takes 33.3 gallons, a saving of 16.7 gallons. From 40 mpg to 50 mpg the use falls from 25 gallons to 20, a saving of only 5 gallons.

That is part of the reason why litres per 100 kilometres, which measures fuel used per distance rather than distance per fuel, is used in much of Europe.

Working out your real mpg

The most reliable method is the full-tank method. Fill the tank to the click on the pump, and note the mileage or reset the trip meter. Drive as normal. At the next fill, again fill to the click, and note the litres added and the miles driven since the last fill.

Suppose you drove 412 miles and the pump added 38.6 litres. Convert litres to gallons by dividing by 4.54609: 38.6 divided by 4.54609 is 8.49 gallons. Then divide the miles by the gallons: 412 divided by 8.49 is 48.5 mpg.

One tank can mislead, because the pump may stop at different points. Repeat the measurement over several fills and take the total miles divided by the total gallons. The trip computer in the car gives a useful guide, but it can read slightly high or low, so compare it with the tank method once.

Converting between mpg and litres per 100 km

The conversion between UK mpg and litres per 100 km uses a constant of 282.48. Divide 282.48 by the mpg to get litres per 100 km, or divide it by the litres per 100 km to get mpg. The constant comes from 4.54609 litres per gallon, multiplied by 100 and divided by 1.609344 kilometres per mile.

Examples: 30 mpg is 282.48 divided by 30, which is 9.42 litres per 100 km. 40 mpg is 7.06. 50 mpg is 5.65. 60 mpg is 4.71. Going the other way, a car using 5.5 litres per 100 km does 282.48 divided by 5.5, which is 51.4 mpg.

For the American gallon, the constant is 235.21, and US miles per gallon figures are about 17 per cent lower than the UK figures for the same car. Our 48.5 mpg result is 40.4 in US gallons. If you see a foreign review with a mpg figure for the car you own, check which gallon it uses before comparing.

Fuel cost per mile

Cost per mile equals the price of a gallon divided by the mpg. A gallon costs 4.54609 times the price per litre. At 140 pence per litre, that is 636.5 pence, or £6.37.

For a car doing 45 mpg, the cost per mile is 636.5 divided by 45, which is 14.1 pence. For a car doing 30 mpg, it is 21.2 pence. For one doing 60 mpg, it is 10.6 pence.

An easy way to get the figure is price per litre times 4.54609, divided by mpg. 140 times 4.54609 is 636.5, then divide. If you want to compare cars in pounds, multiply by the miles you drive in a year. At 12,000 miles a year and 140 pence per litre, a car doing 45 mpg uses 1,212 litres, costing £1,697, while a car doing 55 mpg uses 992 litres, costing £1,389. That is a difference of about £308 a year.

Cost of a journey

For a particular trip, divide the miles by the mpg to get gallons, convert to litres, and multiply by the price. A 200 mile journey in a car doing 45 mpg uses 200 divided by 45, which is 4.44 gallons. That is 4.44 times 4.54609, which is 20.2 litres. At 140 pence a litre, the cost is £28.29.

If you are sharing the cost with passengers, divide by the number of people in the car. Four people sharing £28.29 pay about £7.07 each. Add in other costs, such as tolls and parking, if you are working out a fair share.

For business mileage, HMRC sets an approved rate, which is not taxed, for employees using their own cars, which is intended to cover fuel and wear. Check the current rates on GOV.UK because they are separate from the real cost of fuel.

Why the brochure figure is rarely what you get

Official fuel economy figures come from laboratory tests under standard conditions. Real driving involves cold starts, traffic, hills, heating, air conditioning, heavy loads and faster speeds, all of which reduce economy. It is common to get 10 to 25 per cent less than the official figure, and more in short journeys in cold weather.

For that reason, use the official number for comparing cars and your own tank-by-tank result for budgeting. If the official figure is 55 mpg and you consistently get 46, use 46 in your sums.

Plug-in hybrids are a special case. The official figure assumes some driving on the battery, and a driver who rarely charges will see a much lower figure, close to a petrol car of the same weight.

Ways to improve your economy

The biggest single factor you control is how you drive. Smooth acceleration and anticipating the road so that you brake less use less fuel. Fuel use rises noticeably at higher speeds because air resistance increases with the square of speed, so a steady 60 mph on a motorway uses less than 80 mph for the same distance.

Keep your tyres at the pressure given in the car's handbook, since low pressure increases rolling resistance. Remove roof boxes and racks when you are not using them, as they add drag, and clear heavy items from the boot.

Combine short trips into one, because a warm engine is more efficient than a cold one. Track your mpg each month, and if it drops suddenly, have the car checked, as a fault or a dragging brake can waste fuel.

Tools mentioned in this guide

Frequently asked questions