One car, two mpg figures
The Ford Focus is sold on both sides of the Atlantic, and if you look up its fuel economy in both countries you will find two numbers that seem to disagree. Part of the gap is that the EPA and WLTP tests differ. But a fixed 20% of it is simply the gallon. An imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres. Fill a car with one imperial gallon and it drives 20.095% further than on one US gallon, because it has 20.095% more fuel.
So 40 mpg in America is 48.04 mpg in Britain, and 40 mpg in Britain is 33.31 mpg in America. Neither figure is wrong. This is the confusion the US vs UK gallon guide exists to untangle, and this converter is the quick fix for it.
The formula
MPG (UK) = MPG (US) × 1.200950
MPG (US) = MPG (UK) ÷ 1.200950
The factor is 4.54609 divided by 3.785411784, the ratio of the two gallons. It is the only conversion on this site that does not involve the mile at all: miles cancel out, and only the gallons remain. That is also why it is a straight multiplication with no reciprocal.
Worked example: reading an imported spec sheet
A used-car listing in Britain for a Japanese-import MPV quotes 30 mpg, taken from an American owner's forum. Is that imperial or US? If the source is American, the car is really doing 36.03 mpg imperial, which is respectable. If someone has already converted it, the car is doing 30 imperial, or 24.98 US, which is poor. The listing does not say, and that ambiguity is worth a 20% swing in running costs.
The safe habit is to convert every foreign figure explicitly and label the result. 30 mpg (US) = 36.03 mpg (UK). Written that way, nobody downstream can misread it.
Why the two gallons exist
The US gallon descends from the English wine gallon of 231 cubic inches, standardised in 1707 and kept by the American colonies. Britain replaced it in 1824 with a new imperial gallon defined by the volume of ten pounds of water, later fixed at 4.54609 litres. America never adopted the 1824 change, and the two have been different ever since. Canada used the imperial gallon until it went metric in the 1970s, which is why very old Canadian mpg figures look British rather than American.
Litres and liters are the same unit and the spelling makes no difference here; it is the gallons that differ.
Mistakes to avoid
- Assuming a figure with no country attached is imperial because you are in Britain, or American because you are in the US. Check the source.
- Applying the 1.2 factor twice, once by hand and once by a website that already did it.
- Treating the converted number as a like-for-like comparison of two official ratings. The gallon is fixed, but the test cycles are not; a converted EPA figure still comes from an EPA test.
- Using 1.2 instead of 1.20095. The rounding error is small, under a tenth of an mpg at 50 mpg, but it is unnecessary.