The imperial gallon problem
Britain sells fuel in litres, measures distance in miles, and rates fuel economy in miles per gallon using a gallon that has not been sold at a pump since the 1990s. The gallon in question is the imperial one, defined since 1985 as exactly 4.54609 litres, and it is about 20% larger than the US gallon of 3.785411784 litres. That size difference is the whole reason this page exists separately from the US one.
A British car advertised at 50 mpg is doing 5.65 L/100km. Run the same 50 through a US-gallon converter and you get 4.70 L/100km, a figure the car cannot achieve. Every online converter that offers a single 'mpg' box without asking which gallon is quietly assuming one or the other, and half of its users are getting the wrong answer.
The formula
L/100km = 282.480936 ÷ MPG (imperial)
The constant is 100 times 4.54609 divided by 1.609344. Compare it with the US constant of 235.214583; the ratio between them, 1.200950, is exactly the ratio of the two gallons. Everything on this page is computed from the 4.54609 definition rather than from a rounded 4.5 or 4.55.
Worked examples from UK figures
A small diesel with a WLTP combined figure of 62.8 mpg: 282.480936 divided by 62.8 is 4.50 L/100km. A petrol family SUV at 38.2 mpg: 7.39 L/100km. A hot hatch at 33 mpg: 8.56 L/100km.
Now the trap in reverse. A UK reader finds a US review of the same SUV quoting 32 mpg and thinks the American car is thirstier. It is not: 32 US mpg is 38.4 imperial mpg, near enough the British figure once the different test cycles are allowed for. The MPG US to UK page does that comparison directly.
Why the UK still uses mpg at all
Habit, mostly, plus the road signs. Distances on British roads are in miles and speed limits are in mph, so a distance-per-fuel unit in miles feels natural even though the fuel itself is bought in litres. The official figures on the government's fuel economy label and on manufacturers' sites are stated in imperial mpg, with L/100km sometimes given alongside. Ireland, which drives on the same side of the road, went fully metric and uses L/100km.
Litres or liters: British sources always write litres, and this page follows them. Nothing in the maths changes with the spelling.
Mistakes to avoid
- Using a converter that does not say which gallon it uses. If it does not say, assume it is American and that your answer is 20% off.
- Comparing the UK combined figure with an EPA combined figure without converting the gallon. They are in different units even though both say mpg.
- Trusting a pre-2018 NEDC figure. Those were markedly optimistic; WLTP figures for the same car are typically 10 to 20% lower, which the ratings guide explains.
- Rounding the gallon to 4.5 litres. That introduces a 1% error, small but avoidable.