Why are UK and US mpg different?

A UK (imperial) gallon is 4.54609 litres while a US gallon is 3.785412 litres. Because the UK gallon is larger, the same car shows a higher mpg figure in the UK.

How do I convert UK mpg to US mpg?

Divide the UK figure by 1.200950 (the ratio of gallon sizes). To convert US mpg to UK mpg, multiply by 1.200950.

Does this tool use the internet?

No — every conversion runs locally in your browser and nothing is uploaded.

What is 40 US mpg in UK mpg?

40 × 1.200950 = 48.04 UK mpg. The same car always shows a higher number in UK gallons because they are larger.

Why is the factor 1.200950?

It is the ratio of gallon sizes, 4.54609 ÷ 3.785412 = 1.200950. The UK gallon is about 20% larger than the US gallon, so UK mpg is about 20% higher for identical real economy.

Does a higher mpg number mean better economy across the two systems?

Not necessarily — you must convert first. A 48 UK mpg car and a 48 US mpg car do NOT have the same fuel economy; the US-rated car is actually more efficient.

Miles per Gallon UK to US Converter

This converter changes fuel economy between UK (imperial) miles per gallon and US miles per gallon, in both directions. It exists because a UK gallon and a US gallon are different sizes, so the same car is rated with different mpg numbers on each side of the Atlantic.

How it works

The imperial gallon is 4.54609 L and the US gallon is 3.785411784 L, so the conversion is the ratio of those two volumes:

UK mpg = US mpg × (4.54609 ÷ 3.785412) = US mpg × 1.200950
US mpg = UK mpg ÷ 1.200950

The relationship is exact and bidirectional — enter a value in either field and the other updates instantly. Because the UK gallon is larger, a UK mpg figure is always the higher of the two for the same real-world economy.

Example

A car is advertised at 35 US mpg and you want the UK figure.

35 × 1.200950 = 42.03 UK mpg.

US mpg	UK mpg
20	24.02
30	36.03
40	48.04
50	60.05

Every conversion runs locally in your browser, with no data sent to a server.