UK MPG to MPG: taking the imperial gallon out of a British figure
What this conversion does
It converts a British imperial miles-per-gallon figure into the US equivalent. Where the opposite page multiplies by 1.20, this one divides by it — or multiplies by 0.8327 — so that a British number can sit beside an American one without flattering the car by a fifth of its value.
Why dividing by 1.20 is the honest direction
The imperial gallon is 4.54609 litres against the US 3.785411784, so every imperial MPG figure carries about 20% of extra gallon inside it. Removing that means dividing by 1.200950. A British car quoted at 60 mpg is 50.0 mpg (US) — still an efficient car, but not the 60 mpg an American reader would assume from the headline number.
MPG (UK) to MPG (US) conversion table
Imperial figures with their American equivalent. The gap is roughly a fifth at every point on the scale, from economy cars to performance models.
| Miles per gallon (UK) | Miles per gallon (US) |
|---|---|
| 10 mpg (UK) | 8.3267 mpg |
| 15 mpg (UK) | 12.4901 mpg |
| 20 mpg (UK) | 16.6535 mpg |
| 25 mpg (UK) | 20.8169 mpg |
| 30 mpg (UK) | 24.9802 mpg |
| 35 mpg (UK) | 29.1436 mpg |
| 40 mpg (UK) | 33.3070 mpg |
| 45 mpg (UK) | 37.4703 mpg |
| 50 mpg (UK) | 41.6337 mpg |
| 60 mpg (UK) | 49.9605 mpg |
Why the 20% matters when you are costing fuel
In everyday driving the gallon size affects more than a headline: it changes range and running-cost estimates by a fifth. A British review reporting 50 mpg as a motorway figure is describing 5.65 litres per 100 km — 56.5 litres over 1,000 km — rather than the 47 litres you get by applying the US constant to the same number. Anyone building a fuel budget, comparing lease costs across markets or estimating how often they will refuel needs the figure in their own gallon first.
Remembering the direction
The imperial gallon is the bigger one, so the imperial figure is always the bigger number. If your converted US figure came out higher than the UK one you started with, you have multiplied where you should have divided. As a check, 1 US gallon is exactly 0.832674 imperial gallons.
Frequently asked questions
What is the exact conversion factor?
Divide the imperial figure by 1.200950, which is the same as multiplying by 0.832674. The factor derives from the exact gallon definitions — 4.54609 ÷ 3.785411784 — and holds for every vehicle and every fuel type.
Why does a British car look so much more efficient than an American one?
Partly the gallon and partly the test cycle. About 20% of the difference is the imperial gallon alone; the rest comes from WLTP and EPA measuring differently. Remove the gallon first with this tool, then treat any remaining gap as a cycle difference rather than a real efficiency advantage.
Does this apply to fuel prices as well as economy?
The gallon conversion applies to any volume, so a UK price per imperial gallon converts to a US price per US gallon by the same 1.200950 ratio. In practice Britain prices fuel per litre, so a litre-to-gallon step is usually needed first.
Is the result ever the same for both gallons?
Only at zero, which the tool rejects because it describes a car that covers no distance. For every real vehicle the imperial figure is exactly 1.200950 times the US figure, so the two are never equal.
My figure is from an older British road test — is it still comparable?
Broadly, but older tests used the NEDC cycle, which was more generous than today's WLTP. The gallon conversion here remains exactly correct; what has changed is the test behind the original number, which typically means an older quoted figure flatters the car further. Treat pre-2017 UK economy figures with extra caution.