Fuel economy units and their equivalents
| Unit | Symbol | In l/100 km |
|---|---|---|
| Litre per 100 kilometres | l/100 km | 1 l/100 kmbase unit |
| Mile per US gallon | mpg | 235.215 l/100 km |
| Mile per imperial gallon | mpg UK | 282.481 l/100 km |
| Kilometre per litre | km/l | 100 l/100 km |
| Gallon per 100 miles | gal/100 mi | 2.35215 l/100 km |
Frequently asked questions
How do you go from mpg to litres per 100 km?
By dividing 235.21 by the number of miles per gallon, if it is the US gallon. Thirty mpg is therefore 7.84 l/100 km, and forty mpg 5.88. For the British imperial gallon, a fifth larger, the constant becomes 282.48: thirty imperial mpg is 9.42 l/100 km. A figure that does not say which gallon leaves twenty per cent of uncertainty.
Why can you not average two fuel economy figures?
Because the average of an inverted scale is not the average of the numbers. A trip at 5 l/100 km followed by an equal trip at 10 l/100 km does average 7.5 l/100 km, since consumption adds up per kilometre. But in miles per gallon the same trips, 47 mpg then 23.5, do not average 35.3 mpg: they average 31.4. That is why American agencies also publish a figure in gallons per 100 miles, which does average.
Do you gain more going from 15 to 20 mpg or from 30 to 40?
From 15 to 20, by a wide margin, although the second jump looks twice as large. Over 100 miles, fifteen mpg burns 6.67 gallons and twenty burns 5, a saving of 1.67 gallons. Thirty mpg burns 3.33 and forty burns 2.5, a saving of 0.83, half as much. An inverted scale flattens differences at the top, which is why replacing the thirstiest vehicle in a household pays more than improving the most frugal one.
How does an electric car compare?
In kilowatt-hours per hundred kilometres, the direct analogue of litres per hundred kilometres: same scale, same direction, lower is better. A small electric car uses around 15 kWh/100 km, a large saloon 20 to 25. The United States prefers MPGe, a mile per gallon equivalent, by declaring that a gallon of petrol is worth 33.7 kilowatt-hours: that is an energy equivalence and not a measure of cost, since a kilowatt-hour and a litre are not bought at the same price.
Where are kilometres per litre used?
In Japan, India, Brazil and part of South-East Asia, where the scale is the English-speaking one, distance per volume, but with metric units. Twenty kilometres per litre is 5 l/100 km, fifteen is 6.67 and ten is 10. The conversion is the same division as for miles per gallon, with 100 as the constant instead of 235.21.
Two units, two different questions
Litres per hundred kilometres answers "what will this trip cost me", and miles per gallon answers "how far can I go on a tankful". These are not two ways of saying the same thing but two questions, and only the first one adds up. Two hundred kilometres at 6 l/100 km then a hundred at 9 makes 21 litres, plain proportion. The same trips in miles per gallon do not add up at all, which is why European labelling rules require consumption rather than distance covered.
The published figure has itself changed without the cars changing. Europe moved in 2018 from the NEDC test cycle to WLTP, closer to real driving, and quoted consumption rose by about twenty per cent overnight. The United States had revised its method the same way in 2008. The unit has nothing to do with it, the measurement protocol moved: comparing a 2016 sheet with a 2020 one is comparing two different exams.