UK MPG to L/100km: the imperial gallon makes the difference
What this conversion does
It converts an imperial miles-per-gallon figure — the one British magazines, dealers and MOT history use — into litres per 100 kilometres. The arithmetic looks almost identical to the US conversion, but the constant is different, and using the wrong one silently shifts every answer by about 20%.
Why the UK constant is 282.5 and not 235.2
An imperial gallon is 4.54609 litres, against 3.785411784 for a US gallon — about 20% larger. A bigger gallon carries a car further, so the distance covered on one gallon rises and the constant in the numerator rises with it: 100 km ÷ 1.609344 km per mile × 4.54609 L per gallon gives 282.480936, shown here as 282.5. Everything else about the conversion is unchanged.
MPG (UK) to L/100km conversion table
British fuel economy figures against their metric equivalent. A car sold as 45 mpg in the UK is 6.28 L/100km, while the same car would be advertised as 37.5 mpg in the United States.
| Miles per gallon (UK) | Litres per 100 km |
|---|---|
| 10 mpg (UK) | 28.2481 L/100km |
| 15 mpg (UK) | 18.8321 L/100km |
| 20 mpg (UK) | 14.1240 L/100km |
| 25 mpg (UK) | 11.2992 L/100km |
| 30 mpg (UK) | 9.4160 L/100km |
| 35 mpg (UK) | 8.0709 L/100km |
| 40 mpg (UK) | 7.0620 L/100km |
| 45 mpg (UK) | 6.2774 L/100km |
| 50 mpg (UK) | 5.6496 L/100km |
| 60 mpg (UK) | 4.7080 L/100km |
UK MPG and US MPG are not the same number
This is the single most common error in transatlantic car comparisons. The same vehicle, with exactly the same engine and the same real consumption, is quoted as 45 mpg (UK) in Britain and 37.5 mpg (US) in America — a ratio of 282.480936 ÷ 235.214583, or 1.20. Neither figure is wrong; they simply count different gallons. Whenever you see an MPG number with no country attached, establish which gallon it means before comparing it with anything.
Where UK figures come from
British official economy figures are produced on the WLTP cycle and reported in imperial MPG, and the older NEDC-derived numbers still appear on many used listings. If you are checking a used car, treat the quoted figure as a laboratory result and expect to lose 10–20% in mixed real driving, more in town.
Frequently asked questions
Why is UK MPG higher than US MPG for the same car?
Because the imperial gallon is about 20% larger than the US gallon, so one "gallon" takes the car further. The ratio is exactly 1.200950, derived from the two gallon definitions, and it applies to every car regardless of how efficient it is.
Is the UK still officially using MPG?
Yes. Britain buys fuel in litres but quotes fuel economy in miles per gallon, unlike almost every other European country, and it kept the imperial gallon definition for that purpose. Metric L/100km figures appear alongside, which is why conversions like this one are needed so often there.
How do I convert a UK MPG figure to US MPG?
Divide by 1.20 — or multiply by 0.8327 for precision. So 45 mpg (UK) is 37.5 mpg (US), and 60 mpg (UK) is 50.0 mpg (US). The dedicated MPG (UK) to MPG (US) tool on this site does it in one step.
My car says 45 mpg but I only get 38. Why?
The official test is a laboratory cycle run at fixed temperatures with a fully warmed engine. Cold starts, air conditioning, short town trips and motorway speeds all pull the real number down. A 15% shortfall is completely normal and is not a fault with the car.
Does this converter accept zero or negative values?
No. Zero miles per gallon implies the car consumes fuel without moving, and the conversion is mathematically undefined there, so the tool asks for a positive number instead of showing a meaningless result.