Fuel Economy Converter
Enter an economy figure in whatever unit you have it in and get every other form of it. US and imperial gallons are treated as the different sizes they are.
Equivalent
7.35L/100km
- Litres per 100km
- 7.35
- Miles per US gallon
- 32
- Miles per imperial gallon
- 38.4
- Kilometres per litre
- 13.6
- Miles per litre
- 8.45
- The two mpg figures differ because the gallons do. An imperial gallon is 4.546 litres against 3.785 for a US one, so the same car reads about 20% higher in imperial.
- Miles per gallon is a reciprocal measure, so equal steps in it are not equal savings. Litres per 100km is linear, which is why it can be averaged and mpg cannot.
About fuel economy units
Fuel economy is quoted three ways and the three do not agree. Miles per gallon is distance per unit of fuel, so higher is better. Litres per 100km is fuel per unit of distance, so lower is better. Kilometres per litre is a third form used across much of Asia and Africa.
Worse, "miles per gallon" means two different things. The US gallon is 3.785 litres and the imperial gallon is 4.546, so the same vehicle scores about 20% higher in the UK than in the US without a single thing changing. Comparing a US figure against a UK one without converting is one of the most common mistakes in car reviews.
The deeper point is that mpg is a reciprocal measure, and reciprocal measures do not behave the way people expect. Equal steps up the mpg scale are not equal savings, and a set of mpg figures cannot be averaged. Litres per 100km has neither problem, which is why almost every country that changed measured it that way round.
What it works out
- MPG to litres per 100km, and the reverse
- US and imperial gallons kept separate
- Kilometres per litre and miles per litre
- Why mpg cannot be averaged
The formula
L/100km = 235.215 ÷ mpg (US) L/100km = 282.481 ÷ mpg (imperial)
Both conversions are a division rather than a multiplication, because the two measures are reciprocals of each other. One counts distance per unit of fuel and the other counts fuel per unit of distance.
The constants are just the units collected together. A US gallon is 3.785411784 litres and a mile is 1.609344 kilometres, and 100 times the first divided by the second gives 235.215. Doing the same with the 4.54609 litre imperial gallon gives 282.481.
So 32 mpg US is 235.215 ÷ 32, or 7.35 L/100km. The same car in imperial terms is 282.481 ÷ 7.35, which is 38.4 mpg.
The reciprocal relationship is why mpg misleads. Over 10,000 miles, a car at 10 mpg burns 1,000 gallons and one at 15 mpg burns 667 — a saving of 333. At the other end, 40 mpg burns 250 gallons and 45 mpg burns 222, saving 28. Both look like a five mpg improvement; one saves twelve times as much fuel as the other.
It also means a list of mpg figures cannot be averaged. Two cars at 10 and 50 mpg do not average 30: over the same distance they average 16.7, because the thirsty one dominates the total.
- 235.215
- A hundred US gallons in litres, divided by a mile in kilometres.
- 282.481
- The same constant built from the larger imperial gallon.
- mpg
- Distance per unit of fuel — higher is better.
- L/100km
- Fuel per unit of distance — lower is better.
A worked example
A car quoted at 32 miles per US gallon.
That works out to 7.35 L/100km.
- Litres per 100km
- 7.35
- Miles per US gallon
- 32
- Miles per imperial gallon
- 38.4
- Kilometres per litre
- 13.6
- Miles per litre
- 8.45
Questions
How do I convert mpg to L/100km?
Divide 235.215 by the US mpg figure, or 282.481 by an imperial one. A car at 32 mpg US is 7.35 L/100km; one at 32 mpg imperial is 8.83.
Why are US and UK mpg figures different?
Because the gallons are different sizes. A US gallon is 3.785 litres and an imperial gallon is 4.546, about 20% larger. The same car returns 32 mpg in the US and 38.4 mpg in the UK.
Is a higher or lower number better?
Higher for miles per gallon and kilometres per litre, because those measure distance covered per unit of fuel. Lower for litres per 100km, which measures fuel consumed per unit of distance.
Can I average mpg figures?
No, and doing so gives a figure that is always too optimistic. Two cars at 10 and 50 mpg average 16.7 mpg over the same distance, not 30. Convert to L/100km, average those, and convert back.
Which improvement saves more fuel, 10 to 15 mpg or 40 to 45?
The first, by a long way. Over 10,000 miles it saves 333 gallons against 28 — about twelve times as much — even though both look like a five mpg gain. This is why replacing the thirstiest vehicle in a household usually saves the most.
What is a good fuel economy figure?
It depends entirely on the vehicle and how it is driven, so no number here would be useful. Compare against similar vehicles rather than against an average across all of them.
What is MPGe on an electric car?
A regulatory equivalence figure: the distance an electric car covers on the energy in a gallon of petrol, taken as 33.7 kWh. It is useful for comparing electric cars with each other and misleading for comparing running costs, because electricity and petrol are not priced per unit of energy.
Why does my car never match its official figure?
Official figures come from a standardised test cycle, which is repeatable but not representative. Cold starts, short journeys, town traffic, a loaded car and higher speeds all cost fuel the test does not capture.