MPG (US) vs MPG (UK) vs L/100 km: fuel economy without the marketing spin
Three regions, three different fuel economy numbers, and one weirdly inverted unit that makes cross-shopping cars a maths problem.
Fuel economy is the last consumer number that is still routinely reported in three different units. In the US, cars are rated in miles per US gallon (mpg-US). In the UK, the same car may be rated in miles per Imperial gallon (mpg-UK), and everywhere else in the world it is litres per 100 kilometres (L/100 km). The three numbers all describe the same car but they are not the same value — and one of them runs in the opposite direction to the other two.
The three factors
1 US gallon = 3.785411784 L > 1 Imperial gallon = 4.54609 L > 1 mile = 1.609344 km
From that: **mpg-US → L/100 km** is `235.2145833 / mpg`. **mpg-UK → L/100 km** is `282.4809363 / mpg`. **mpg-US → mpg-UK** is multiplied by 1.20095 (because the Imperial gallon is 20.1% larger). This last factor is why UK marketing brochures always look better than their US equivalents: the same physical car gets a bigger mpg number in the UK simply because a UK gallon is bigger.
Worked example: a real EPA number
A 2024 Toyota Corolla Hybrid is EPA-rated at 47 mpg combined. In the UK the same drivetrain achieves 47 × 1.20095 = 56.4 mpg (Imperial). In L/100 km terms both convert to 5.0 L/100 km — the honest, apples-to-apples number.
Why L/100 km flips your intuition
Miles per gallon is a *bigger is better* metric — the further you travel on the same fuel, the higher the number. L/100 km is *smaller is better* — the less fuel you use to cover the same distance, the lower the number. This flips two useful intuitions.
First, it exposes diminishing returns. Going from 20 mpg to 25 mpg saves the same amount of fuel over 10,000 miles as going from 33 mpg to 50 mpg — but the L/100 km numbers make that obvious: 20 mpg = 11.76 L/100km, 25 mpg = 9.41, a saving of 2.35 L/100km, versus 33 mpg = 7.13 to 50 mpg = 4.70, a saving of 2.43. Nearly identical.
Second, it makes fleet averages arithmetic. If half your fleet does 8 L/100 km and half does 12 L/100 km, the fleet average is 10 L/100 km. In mpg you cannot just average — you have to convert to L/100 km, average, then convert back.
Regional oddities
Canada switched from mpg-UK to L/100 km in the 1970s but many drivers still convert to mpg mentally — usually the Imperial version. Australia and New Zealand use L/100 km on every new car sticker. Japan reports km/L (kilometres per litre), which is L/100 km inverted and multiplied by 100.
Five-second cheat sheet
US mpg × 0.4251 = km/L. US mpg × 1.20095 = UK mpg. L/100 km × US mpg = 235.21. If you cannot remember the factor, remember that a UK gallon is roughly 20% bigger, so UK mpg is always the flattering number. And when you see a manufacturer quote a fuel economy improvement, convert both numbers to L/100 km before deciding whether the marketing team is exaggerating.
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FAQ
How many US mpg is 5 L/100 km?+
235.2146 / 5 = 47.04 mpg (US).
Why is UK mpg higher than US mpg for the same car?+
Because an Imperial gallon (4.546 L) is about 20% larger than a US gallon (3.785 L). The car travels the same distance on the same fuel; the unit is just bigger.
Is L/100 km the same as km/L?+
No — they are reciprocals. 10 km/L = 10 L/100 km, 5 km/L = 20 L/100 km.
Which unit should I use to compare cars?+
L/100 km. It is linear in fuel use and averages correctly across a fleet, unlike mpg.
This article was written by The Turbo Unit Converter engineering desk (Metrology & tooling) and last reviewed on 2026-06-10 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.