MPG to UK MPG: why one car has two different economy figures
What this conversion does
It converts a US miles-per-gallon figure into the imperial figure British buyers would see for the same car. Nothing about the vehicle changes — the engine, the consumption and the distance travelled are identical. The only difference is the size of the gallon used to measure the fuel.
The 1.20 ratio, and where it comes from
An imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres, a ratio of exactly 1.200950. Because the imperial gallon is bigger, one "gallon" of fuel takes the car about 20% further, so the imperial MPG figure is about 20% higher. The ratio is constant: it does not matter whether the car does 20 MPG or 60 MPG, the UK number is always 1.20 times the US one.
MPG (US) to MPG (UK) conversion table
The same car described in both gallons. Multiply any US figure by 1.20 to get the British one; divide a British figure by 1.20 to come back.
| Miles per gallon (US) | Miles per gallon (UK) |
|---|---|
| 10 mpg | 12.0095 mpg (UK) |
| 15 mpg | 18.0142 mpg (UK) |
| 20 mpg | 24.0190 mpg (UK) |
| 25 mpg | 30.0237 mpg (UK) |
| 30 mpg | 36.0285 mpg (UK) |
| 35 mpg | 42.0332 mpg (UK) |
| 40 mpg | 48.0380 mpg (UK) |
| 45 mpg | 54.0427 mpg (UK) |
| 50 mpg | 60.0475 mpg (UK) |
| 60 mpg | 72.0570 mpg (UK) |
Why this trips up so many comparisons
A British review praising a car for reaching "50 mpg" against an American review calling the same model "41 mpg" looks like a contradiction, and is routinely reported as one. Both are correct. The failure mode is serious in practice: if you convert a UK figure with the US constant, you understate consumption by 20%, and if you plan a fuel budget from a US figure converted as though it were imperial, you will overestimate range by the same margin.
Which figure to quote
Quote the gallon of your audience and say which one it is. In a mixed forum, writing "45 mpg (UK)" rather than "45 mpg" costs four characters and removes an entire class of argument. If a manufacturer quotes a single figure without qualifiers, treat it as the local gallon for wherever the material was published.
Frequently asked questions
How much bigger is a UK gallon than a US gallon?
About 20%. The imperial gallon is exactly 4.54609 litres and the US gallon exactly 3.785411784, giving a ratio of 1.200950. That single number is the entire difference between a UK and a US MPG figure for the same car.
Why does Britain still use imperial gallons for fuel economy?
Britain sells fuel in litres and drives in miles, and kept imperial MPG for economy figures when it metricated most other measurements. It is now one of very few countries quoting fuel economy in miles per gallon, which is why the distinction matters so much for British motoring content.
Does the ratio change for diesel or hybrid cars?
No. The ratio comes only from the two gallon definitions, so it is exactly the same 1.200950 for petrol, diesel, hybrid and plug-in hybrid alike. Only the size of the gallon in the denominator is being changed.
My car shows different economy in the UK and US — is something wrong?
Nothing is wrong. If the two figures differ by about 20%, they are the same measurement in different gallons. If they differ by a different amount, then the test cycles differ too — WLTP in Britain and the EPA cycle in America rarely produce identical results even after conversion.
Can I convert the other way on this site?
Yes, the MPG (UK) to MPG (US) converter does the reverse, dividing by 1.20. Between the two pages all four fuel units on the site are reachable from each other.