Miles per Gallon (US) to Miles per Gallon (UK) Converter | mpg (US) to mpg (UK)

What this converter does

A car's fuel economy in miles per gallon depends on whose gallon you mean, and the US and the UK do not use the same one. This converter takes an economy figure in US mpg and gives you the equivalent in UK mpg. Enter the US figure and read off the UK one, so two numbers that look like they should match can finally be compared fairly.

Why the two mpg figures differ

The mile is the same on both sides of the Atlantic, but the gallon is not. A US gallon is about 3.785 litres, while a UK gallon, the Imperial one, is about 4.546 litres, roughly 20 percent larger. Since a UK gallon holds more fuel, a car goes further on one, so the same car always scores a higher number in UK mpg than in US mpg. It is not more efficient, it just gets measured against a bigger gallon. That is the whole reason a British car's economy can look so much better than an American one until you convert.

The conversion factor

Because the difference is purely the size of the gallon, the conversion is a single multiplication:

mpg (UK) = mpg (US) × 1.20095

That 1.20095 is just the ratio of the two gallons. Every US mpg figure comes out about 20 percent higher when expressed in UK mpg.

A worked example

Say a car is rated at 30 mpg in the US and you want that figure the way it would be quoted in the UK.

• 30 × 1.20095 = 36 mpg (UK)

So a 30 mpg US car would be advertised as about 36 mpg in the UK. Same car, same fuel, same distance, just a bigger gallon doing the dividing. Knowing that stops you from thinking the UK version is the more economical car when it is the very same one.

Common mpg (US) values in mpg (UK)

As an anchor, 30 mpg US is about 36 mpg UK, and a frugal 50 mpg US comes out near 60 mpg UK.

Where this matters

The main case is comparing cars across the two markets. A US review quotes mpg one way and a UK review the other, and without converting you would be comparing two different gallons and reaching the wrong conclusion. It matters when reading the specs of a model sold on both sides, when importing a car, or simply when an American and a British figure land side by side and you need them on the same footing. The number that looks bigger is not always the better car, and this conversion is what tells them apart.

Questions people ask

How do I convert US mpg to UK mpg?

Multiply the US mpg figure by 1.20095. The difference comes entirely from the UK gallon being about 20 percent larger than the US gallon.

Why is the UK mpg number higher?

A UK gallon holds more fuel than a US gallon, so a car travels further on one. The same car therefore scores a higher mpg in UK terms, without being any more efficient.

What is 30 US mpg in UK mpg?

About 36 mpg. It is a useful anchor for keeping the roughly 20 percent gap in mind.

References

1. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (gallon definitions and unit conversions). https://www.nist.gov/pml/special-publication-811
2. U.S. Department of Energy and EPA, fueleconomy.gov (fuel economy ratings and information). https://www.fueleconomy.gov/