The US liquid gallon is 3.785411784 litres and the imperial gallon used in the UK is 4.54609 litres, so an imperial gallon is about 20 per cent larger. A car doing 30 miles per US gallon does 36.03 miles per imperial gallon and consumes 7.84 litres per 100 km — three numbers describing one car, none of which converts to another by a factor anyone remembers.
This is the rare case where knowing which unit you have matters more than the arithmetic. Both are called a gallon, both are abbreviated gal, and nothing in a printed figure tells you which one was meant.
How far apart are the two gallons?
The imperial gallon is 1.20095 US gallons, a ratio that shows up everywhere fuel is discussed across the Atlantic. It exists because the two definitions came from different places: the US gallon descends from the English wine gallon of 1707, and the imperial gallon was defined in 1824 as the volume of ten pounds of water.
| Litres | US gallons | Imperial gallons |
|---|---|---|
| 1 | 0.2642 | 0.2200 |
| 10 | 2.6417 | 2.1997 |
| 20 | 5.2834 | 4.3994 |
| 50 | 13.2086 | 10.9985 |
| 60 | 15.8503 | 13.1982 |
| 100 | 26.4172 | 21.9969 |
What does that do to a fuel economy figure?
It moves it by a fifth. The same car, measured identically, scores 20 per cent higher in imperial mpg than in US mpg, purely because the gallon it is divided by is bigger.
| US mpg | Imperial mpg | L/100 km |
|---|---|---|
| 20 | 24.02 | 11.76 |
| 25 | 30.02 | 9.41 |
| 30 | 36.03 | 7.84 |
| 35 | 42.03 | 6.72 |
| 40 | 48.04 | 5.88 |
| 50 | 60.05 | 4.70 |
So a British car advertised at 48 mpg and an American car advertised at 40 mpg are the same car. Comparing the headline numbers directly makes the British one look 20 per cent better than it is.
Why is litres per 100 km the more honest measure?
Because it scales linearly with what you are actually buying. Miles per gallon is an inverted measure, so equal steps in mpg are not equal steps in fuel used, and the effect is severe at the bottom of the range.
Going from 15 to 20 US mpg saves 3.92 L/100 km. Going from 40 to 50 saves 1.18 L/100 km. The second improvement looks twice as big in mpg and saves a third as much fuel. If you are choosing between two vehicles to replace, the older thirsty one is almost always the better swap, and mpg hides that.
Which figure is on the sticker?
The one the local regulator requires, measured on the local test cycle — which is a second, larger source of difference. US EPA figures and European WLTP figures come from different drive cycles, so even converted to the same unit they are not measuring quite the same thing.
Treat a converted figure as comparable in unit but not in method. It tells you what the number would have been called elsewhere, not what the car would have scored on that test.
What does the difference cost over a year?
Take 12,000 miles a year in a car doing 30 US mpg. That is 400 US gallons, or 1,514.16 litres. The same car described as 36.03 imperial mpg burns 333.06 imperial gallons — the same 1,514.16 litres, counted with a different scoop.
The error to avoid is planning in one unit and buying in the other. Assume 30 imperial mpg when the true figure is 30 US mpg and you budget for 1,261.8 litres against an actual 1,514.16 — a shortfall of 252 litres, or a fifth of the fuel bill. Across a fleet that is the whole margin.
Which figure does a rental quote?
The one from the market the car was sold into, which is not always the market you are driving in. A car rented in Ireland may carry a UK-market brochure figure in imperial mpg while the pump sells litres and the road signs are in kilometres, so three systems are in play on one journey.
The reliable move is to convert everything to litres per 100 km at the start and stop switching. It is the only one of the three that behaves linearly, it matches what the pump charges, and it is what the on-board computer is most likely already displaying.
Questions people ask
Which gallon does the converter use? Ours converts litres to US gallons, and the page states the imperial figure alongside so the difference is visible rather than assumed.
Is there a third gallon? The US dry gallon, at 4.40488 litres, used for grain and fruit. It is not used for liquids and never for fuel.
How big is a barrel of oil? Forty-two US gallons, or 158.987 litres. It is a unit of trade rather than a container anyone ships in.
Do the UK and US differ on the pint too? Yes, and by the same ratio: an imperial pint is 568.26 ml and a US pint is 473.18 ml. A pint of beer is a fifth larger in London than in New York.
Why does my car report better economy than the sticker? Because the on-board computer measures your driving and the sticker reports a standard test cycle. Steady motorway running beats the cycle; short cold trips lose to it badly.
Is the imperial gallon still legal for trade? Fuel is sold in litres across the UK and Ireland. The imperial gallon survives in economy figures and in conversation rather than at the pump, which is why the mpg number and the price on the display are in different systems.
Check which gallon before you compare anything. Litres to gallons shows both, miles to kilometres handles the distance half of an economy figure, and the volume calculator does tanks and drums in either system.