Which gallon?
A US gallon is 3.785 litres. An imperial gallon is 4.546. They differ by just over twenty per cent, and both are called "mpg".
The second-ranking result for this query contains no occurrence of the word "imperial" anywhere on the page. A visitor in Britain enters the mpg figure from their car's display, gets an answer a fifth out, and is given no reason to doubt it. That is a silent error, which is the worst kind.
So the unit selector here is the first thing on the page rather than a footnote, and when you enter a figure in one kind of gallon the result tells you what it would have meant in the other. If your car was sold in the UK, its mpg is almost certainly imperial.
The conversion is division, not multiplication
Miles per gallon and litres per hundred kilometres run in opposite directions: higher is better in one and worse in the other. The conversion is 235.215 divided by US mpg, or 282.481 divided by imperial mpg.
Because it is a division rather than a multiplication, the difference between 20 and 25 mpg is much larger in fuel terms than the difference between 45 and 50, which is not obvious from the mpg figure alone. That is the practical argument for L/100km, and why most of the world uses it.
Official figures are optimistic
Real-world consumption in Europe runs about twenty per cent above the official WLTP figure — 21.1 per cent for petrol and 17.1 per cent for diesel, measured by the European Commission across a sample of 740,000 vehicles registered in 2022.
In the United States, the average model-year 2024 new vehicle achieves 27.2 mpg in real-world use, a record high, against a higher laboratory figure. Whichever side of the Atlantic you are on, budgeting on the brochure number will leave you short. Enter what your car actually returns if you know it.
Things people ask
US or imperial gallons?
If your car was sold in the United Kingdom, imperial. If it was sold in the United States, US. The two differ by twenty per cent, so it is worth being sure.
What currency does it use?
None in particular. Enter the fuel price in whatever you pay in and the total comes back in the same units.
Should I use my car's official mpg?
Only as a starting point. Real-world consumption runs around a fifth worse than the official European figure, so the trip will cost more than the brochure suggests.