Units explained
Why UK and US MPG are different
They use different gallons. A UK imperial gallon is 4.54609 litres; a US gallon is 3.785411784 litres. A mile is the same, so the same car returns a different MPG number in each country. The same number also describes a different fuel consumption.
The quick comparison
40 MPG UK
7.06 L/100 km
40 MPG US
5.88 L/100 km
40 MPG UK is about 20% higher consumption than 40 MPG US, because the imperial gallon is larger. A UK figure is not automatically better than a US figure with the same number; the label is using a different gallon.
The exact conversion
The two MPG systems can be converted with a ratio. One imperial gallon is about 1.20095 US gallons, so:
MPG (UK) = MPG (US) × 1.20095 MPG (US) = MPG (UK) ÷ 1.20095 1 UK MPG ≈ 0.8327 US MPG 1 US MPG ≈ 1.20095 UK MPG
For L/100 km, the constants differ because the gallon is in the numerator:
MPG (UK) = 282.480936 ÷ (L/100 km) MPG (US) = 235.214583 ÷ (L/100 km)
The constants are not arbitrary. They combine the gallon volume with 1 mile = 1.609344 km. The UK constant is larger because the imperial gallon is larger.
The same MPG number in both systems
Read each row as the L/100 km you get if the MPG figure is interpreted in that system.
| MPG | L/100 km as MPG UK | L/100 km as MPG US | US MPG equivalent of UK figure |
|---|---|---|---|
| 30 | 9.42 | 7.84 | 24.98 |
| 35 | 8.07 | 6.72 | 29.14 |
| 40 | 7.06 | 5.88 | 33.31 |
| 45 | 6.28 | 5.23 | 37.47 |
| 50 | 5.65 | 4.7 | 41.63 |
| 55 | 5.14 | 4.28 | 45.8 |
| 60 | 4.71 | 3.92 | 49.96 |
Why it matters in practice
Vehicle comparisons: a US window-sticker figure and a UK quoted figure are not directly comparable even before test-cycle differences. Convert to the same gallon, or convert both to L/100 km.
Fuel cost: UK fuel is priced per litre, but the car may be described in MPG UK. Multiplying pence per litre by MPG directly is a unit error. Convert to litres used first.
Imported cars and rentals: a car that returns 50 MPG US is about 60.05 MPG UK. It has not become more efficient because the number changed. The unit changed.
EV comparisons: MPG and MPGe are different constructs. MPGe is an energy-equivalence rating for electric vehicles, not a liquid-fuel measurement. Do not convert MPGe to MPG with the gallon ratio.
Test cycles are a separate issue
Common mistakes
- Treating MPG as a universal unit without asking which gallon is meant.
- Using the UK constant for a US MPG figure, or the reverse.
- Comparing a UK MPG with a US MPG and concluding one car is better without converting.
- Multiplying a pence-per-litre price by MPG instead of calculating litres used.
- Applying the UK/US MPG ratio to MPGe, which is a different energy equivalent.
Convert a real figure
The converter accepts L/100 km, MPG UK, MPG US and km/L, and names the system in the result.
Open the fuel efficiency converterSources
- NIST — unit conversion — US customary and metric relationships.
- UK Weights and Measures Act 1985 — The statutory framework for UK units, including the imperial gallon.
- fueleconomy.gov — US fuel-economy ratings — US EPA label values and test-cycle context.
Page last reviewed: . Formulas are covered by automated tests. Prices are inputs, not live data.