Fuel Economy Converter (mpg US/UK, km/L, L/100km)

Why mpg is bigger in the UK

1 UK / imperial gallon is 4.5461 L; the US gallon is only 3.7854 L. So a car running at 30 mpg in the UK uses one gallon (4.55 L) per 30 miles; a 30 mpg US car uses one gallon (3.79 L) per 30 miles. The UK car is going further per liter — UK mpg numbers look better for the same vehicle. Multiply UK mpg by 0.8327 to get US mpg.

This is the most common cross-Atlantic confusion in car reviews. When you see a UK auto magazine quoting 50 mpg, that's about 42 mpg in US terms — and roughly 5.6 L/100km.

L/100km vs mpg — they're inverted

L/100km is fuel used per fixed distance. Smaller is better. Most of Europe and Japan use this.

mpg is distance traveled per fixed fuel. Bigger is better. The US, UK, and Canada use this.

Because of the inversion, equal-percent improvements look very different. Going from 6 to 5 L/100km is a 17% efficiency gain — but going from 39 to 47 mpg (the equivalents) is a 21% gain by mpg math. Always compare on the same metric.

Quick reference

30 mpg (US) ≈ 36 mpg (UK) ≈ 12.75 km/L ≈ 7.84 L/100km. 50 mpg (US) ≈ 60 mpg (UK) ≈ 21.25 km/L ≈ 4.7 L/100km. A modern compact ICE car gets ~30-40 mpg US / 6-8 L/100km. Hybrids get 50+ mpg US / under 5 L/100km. EVs use kWh/100km or kWh/mi instead — different metric entirely.