mpg (US) · mpg (UK) · L/100 km · km/L
Fuel economy converter
Type a fuel economy figure in any unit to see it in the other three. US and UK miles per gallon are different numbers for the same car, because the gallons differ.
- mpg (US)32.00what you entered
- mpg (UK)38.43miles per UK gallon
- L/100 km7.35litres per 100 km
- km/L13.60km per litre
That is 3.13 US gallons (11.83 litres) per 100 miles, and 7.35 litres per 100 km.
Two gallons and a litre
A US gallon is 3.785 litres and a UK (imperial) gallon 4.546 litres, so a car does about 20% more miles per UK gallon than per US gallon. Litres per 100 km turns the figure upside down: it is fuel per distance, so a lower number is better. km/L is distance per litre, the right way up again.
The mpg illusion
Because mpg is distance per fuel, equal steps in mpg are not equal savings. Richard Larrick and Jack Soll showed in Science in 2008 that people judge fuel savings as if mpg were a straight line, and suggested quoting fuel per distance instead. Two car swaps over 10,000 miles show why:
| Swap (mpg US) | Fuel used | Fuel saved | L/100 km |
|---|---|---|---|
| 10 to 20 mpg | 1,000 to 500 gal | 500 gal (1,893 L) | 23.5 to 11.8 (saves 11.8) |
| 25 to 50 mpg | 400 to 200 gal | 200 gal (757 L) | 9.4 to 4.7 (saves 4.7) |
The 10-mpg gain saves 500 gallons; the 25-mpg gain, which looks bigger, saves only 200. In L/100 km the savings (11.8 against 4.7) line up with the fuel saved, which is why fuel used per distance compares cars more fairly.
Comparison table
| mpg (US) | mpg (UK) | L/100 km | km/L | US gal per 100 mi |
|---|---|---|---|---|
| 10 | 12.0 | 23.5 | 4.3 | 10.00 |
| 15 | 18.0 | 15.7 | 6.4 | 6.67 |
| 20 | 24.0 | 11.8 | 8.5 | 5.00 |
| 25 | 30.0 | 9.4 | 10.6 | 4.00 |
| 30 | 36.0 | 7.8 | 12.8 | 3.33 |
| 35 | 42.0 | 6.7 | 14.9 | 2.86 |
| 40 | 48.0 | 5.9 | 17.0 | 2.50 |
| 50 | 60.0 | 4.7 | 21.3 | 2.00 |
| 60 | 72.1 | 3.9 | 25.5 | 1.67 |
Sources
- Larrick RP, Soll JB. The MPG Illusion. Science 2008;320(5883):1593–1594. doi:10.1126/science.1154983
- US gallon: 231 cubic inches, from NIST Handbook 44, Appendix C; with the inch defined as exactly 2.54 cm, exactly 3.785411784 litres.
- UK (imperial) gallon: exactly 4.54609 litres, from the Weights and Measures Act 1985, Schedule 1.
- Mile: exactly 1.609344 km; 1 mpg (US) = 0.4251437 km/L, from NIST Special Publication 811, Appendix B.8.
Questions
Is 30 mpg the same in the US and the UK?
No. The UK gallon holds more fuel, so the same car shows a higher figure in UK mpg: 30 mpg (US) is 36.0 mpg (UK), and 30 mpg (UK) is 25.0 mpg (US). Check which gallon a figure uses before comparing cars.
How do I convert mpg to L/100 km by hand?
Divide 235.215 by mpg (US), or 282.481 by mpg (UK). The same division works in reverse: 235.215 divided by L/100 km gives mpg (US). For km/L, divide 100 by L/100 km.
Which unit is best for comparing cars?
Fuel used per distance, such as L/100 km or US gallons per 100 miles, because halving it halves your fuel bill. With mpg, the same gain saves far more fuel on a thirsty car than on an efficient one.