US MPG, UK MPG and L/100 km
Updated 8 November 2026. Every figure on this page comes from the same formulas as our calculators.
The same car can be quoted at 30 MPG in the US and 36 mpg in the UK without anything being wrong. The units are different, and so are the official tests. This guide explains both.
Two different gallons
- A US gallon is 3.785 litres.
- An imperial (UK) gallon is 4.546 litres, about 20 % bigger.
A bigger gallon takes you further, so the UK mpg number is always higher for the same car: UK mpg = US MPG × 1.201.
Litres per 100 km works the other way round
MPG measures distance per fuel (higher is better). L/100 km measures fuel per distance (lower is better). The conversions are:
- L/100 km = 235.2 ÷ US MPG
- L/100 km = 282.5 ÷ UK mpg
| US MPG | UK mpg | L/100 km | km/L |
|---|---|---|---|
| 30.0 | 36.0 | 7.84 | 12.8 |
| 33.3 | 40.0 | 7.06 | 14.2 |
The fuel economy converter handles any value.
Why fuel per distance is the fairer comparison
Fuel used falls less and less as MPG rises. Over 100 miles, a 10 MPG truck uses 10 gallons and a 20 MPG truck uses 5, a saving of 5 gallons. A 30 MPG car uses 3.33 gallons and a 40 MPG car 2.5, a saving of under a gallon. The same 10 MPG gap is worth far more at the bottom of the scale. The fuel cost comparison turns that into money per year.
Official tests: EPA, NEDC and WLTP
- EPA (US): laboratory tests adjusted to reflect real-world driving, published as city, highway and combined MPG.
- NEDC: the old European test, used for UK figures until the switch to WLTP. Its figures are noticeably optimistic.
- WLTP: the current test, introduced for new car types from September 2017 and all new cars from September 2018. It is longer and faster than NEDC, so its figures are lower and closer to real driving.
Because the tests differ, an NEDC figure, a WLTP figure and an EPA figure for the same car are not comparable even after converting the units. That is why our UK pages keep NEDC and WLTP in separate tables, and why our US pages only show EPA ratings.
Electric cars
Electric cars are rated differently: the US uses MPGe (miles per gallon equivalent, where 33.7 kWh counts as one gallon) and kWh per 100 miles; the UK uses miles per kWh and range. They measure efficiency, not fuel, so they cannot be converted into a petrol MPG.