Fuel economy is measured in different units around the world, and they do not all work the same way. In the United States it is miles per gallon (MPG), in the UK it is also MPG — but with a bigger gallon — while most of Europe, Australia and many other countries use litres per 100 kilometres (L/100km), and parts of Asia, Latin America and Africa use kilometres per litre (km/L). Here is how to read and convert each one.
Distance per fuel vs fuel per distance
The most important thing to understand is that the units point in opposite directions:
- MPG and km/L measure distance per unit of fuel. Higher is better.
- L/100km measures fuel used per distance. Lower is better.
L/100km has one big advantage: it scales directly with fuel cost. Halving L/100km halves your fuel bill. With MPG the relationship is not linear — improving from 10 to 20 MPG saves far more fuel than improving from 40 to 50 MPG over the same distance.
US gallons vs UK (imperial) gallons
A US gallon is 3.785 litres; an imperial (UK) gallon is 4.546 litres. Because the UK gallon is about 20% larger, the same car shows a higher MPG figure in the UK. To convert:
- UK MPG = US MPG × 1.201
- US MPG = UK MPG × 0.833
Unless stated otherwise, MPG figures on CarDetails.org are US MPG.
Conversion formulas
- L/100km = 235.215 ÷ US MPG
- US MPG = 235.215 ÷ L/100km
- km/L = US MPG × 0.4251
- US MPG = km/L × 2.352
- L/100km = 100 ÷ km/L
- L/100km = 282.481 ÷ UK MPG
Quick reference table
| US MPG | UK MPG | km/L | L/100km |
|---|---|---|---|
| 15 | 18.0 | 6.4 | 15.7 |
| 20 | 24.0 | 8.5 | 11.8 |
| 25 | 30.0 | 10.6 | 9.4 |
| 30 | 36.0 | 12.8 | 7.8 |
| 35 | 42.0 | 14.9 | 6.7 |
| 40 | 48.0 | 17.0 | 5.9 |
| 50 | 60.0 | 21.3 | 4.7 |
Worked example: a car rated at 35 US MPG uses 235.215 ÷ 35 = 6.7 L/100km, travels 35 × 0.4251 = 14.9 km per litre, and would be advertised at about 42 MPG in the UK.
Official figures and real-world economy
Official fuel-economy numbers come from standardised laboratory tests, and different regions use different tests:
- EPA (United States) — generally considered the closest to real-world driving.
- WLTP (Europe and many other markets since 2017–2018) — more realistic than the test it replaced, but often more optimistic than EPA.
- NEDC (older European figures) — the least realistic; real-world consumption is often noticeably higher.
Because the tests differ, compare cars using figures from the same test wherever possible. Your own results will depend on traffic, speed, climate, load, tyre pressure and driving style.
Electric cars
EV efficiency is usually given as kWh per 100 km, kWh per 100 miles, miles per kWh, or as MPGe in the US. MPGe expresses the energy an EV uses in terms of the energy in a gallon of petrol (33.7 kWh), so it can be compared with a petrol car’s MPG — but the cost per mile depends on your electricity price.
Convert instantly
Every car page on CarDetails.org has a fuel-economy converter in the sidebar: type an MPG or km/L figure and it shows the equivalents instantly. Find a car to try it.