UK MPG vs US MPG: Why the Numbers Are Different

If you have ever read a British car review claiming 55 mpg and then looked at the US EPA rating for the same model showing only 45 mpg, you might have assumed the two figures were measuring different test cycles or that something was wrong with the American version. In most cases, neither explanation is correct. The real answer comes down to a single, frequently overlooked fact: the United Kingdom and the United States use gallons of different sizes.

Two Countries, Two Gallons

The United States gallon, sometimes called the liquid gallon, holds 3.78541 litres. The Imperial gallon used in the United Kingdom holds 4.54609 litres. That difference works out to roughly 20 percent more volume per gallon on the British side.

Because miles per gallon measures how far a vehicle travels on one gallon of fuel, a larger gallon automatically produces a higher number even when the underlying fuel consumption rate is identical. A car burning exactly 6 litres per 100 kilometres will return approximately 47.1 UK mpg or 39.2 US mpg. The car has not changed. Only the unit of measurement has.

This is one of the most persistent sources of confusion in cross-Atlantic car comparisons, and it catches out even experienced automotive journalists who forget to flag which standard they are using.

The Conversion Formula

Converting UK mpg to US mpg is straightforward. The ratio of the two gallons is 4.54609 divided by 3.78541, which equals approximately 1.20095. That number is your conversion factor.

To go from UK mpg to US mpg, divide the UK figure by 1.20095. To go the other direction, multiply the US figure by 1.20095.

So if a British road test reports 50 mpg on the Imperial scale, the equivalent US figure is 50 divided by 1.20095, which gives you 41.6 US mpg. A car rated at 60 UK mpg works out to 60 divided by 1.20095, which equals 49.9 US mpg. The car is the same, the fuel consumption is the same, but the number changes by about a fifth depending on which shore you are standing on.

You can run these calculations quickly using our UK MPG Calculator.

Where the Confusion Gets Costly

The practical problem arises when someone reads a UK automotive review, notes the mpg figure, and then compares it directly to a US-market competitor without adjusting. You might conclude that a European hatchback is meaningfully more efficient than an American sedan when the two are actually identical in real-world consumption.

This happens with imported vehicles too. If you buy a car in the United States that was previously sold in the United Kingdom, any fuel economy figures in the original documentation are in Imperial mpg. Using those numbers for budgeting your fuel costs in the US will leave you disappointed, because the same car will post lower numbers on American pumps, not because it performs worse, but because the gallon you are buying is smaller.

Car enthusiast forums are full of threads where someone insists a particular diesel engine returns extraordinary economy, only for a second commenter to point out the original source was a UK publication. This misunderstanding has probably led more than a few people to make purchasing decisions based on numbers that simply did not apply to them.

A Quick Reference Table

The table below converts common UK mpg figures to their US mpg equivalents and also shows the L/100km figure, which is useful as an independent check.

UK MPG	US MPG	L/100km
40	33.3	7.07
45	37.5	6.27
50	41.6	5.65
55	45.8	5.13
60	49.9	4.71
65	54.1	4.35
70	58.3	4.04

Notice that the gap between UK mpg and US mpg widens in absolute terms as the numbers climb. At 40 UK mpg, the difference is about 6.7 mpg. At 70 UK mpg, the difference is nearly 12 mpg. The percentage stays constant at roughly 20 percent, but the raw number diverges more as efficiency increases.

L/100km as the Universal Benchmark

Litres per 100 kilometres has one significant advantage over both mpg standards: it is the same everywhere. A car that uses 5.5 L/100km in Germany uses 5.5 L/100km in Canada, the United States, and Japan. There is no conversion needed, no gallon size to account for, no ambiguity about which version of the measurement is being used.

For anyone who regularly reads car reviews from multiple countries, L/100km is the cleanest common ground. You can compare a French small car against a Korean crossover against an American pickup truck without first stopping to check which country published the test.

Our MPG to L/100km Converter handles both UK and US mpg inputs, so you can always arrive at that neutral figure regardless of which standard you started with.

The Practical Takeaway

Whenever you read a fuel economy figure, the first question to ask is which gallon it refers to. British publications, including car magazines, manufacturer press releases for UK-market vehicles, and MOT documentation, use Imperial gallons. American publications, EPA window stickers, and US manufacturer specs use US liquid gallons.

If the source does not specify, look at context clues. A figure of 60 mpg or above for a non-hybrid petrol car almost certainly comes from a UK source, since that level of efficiency is uncommon in US EPA testing. A figure below 40 mpg for a small modern hatchback might be a US figure that looks unremarkable but would actually represent solid efficiency on the Imperial scale.

The 20 percent rule is a reliable starting point for quick mental arithmetic. Take any UK mpg figure, subtract a fifth of it, and you have a reasonable approximation of the US equivalent. For precise calculations, divide by 1.20095 or use a dedicated converter to get an exact answer.

Understanding this distinction will not make any car more efficient. It will, however, prevent you from comparing two vehicles on unequal terms and reaching conclusions that do not hold up in real-world use.