MPG converter
UK and US mpg are different, because a UK gallon is 4.546 litres and a US gallon is 3.785. 40 UK mpg is about 33.3 US mpg, or 7.1 litres per 100 km.
- Litres per 100 km
- 7.1
- US mpg
- 33.3
- Km per litre
- 14.2
- Miles per litre
- 8.80
40 UK mpg. UK gallons are 4.546 litres; US gallons are 3.785 litres.
UK mpg conversion table
| UK mpg | Litres per 100 km | US mpg | Km per litre |
|---|---|---|---|
| 20 mpg | 14.1 | 16.7 | 7.1 |
| 25 mpg | 11.3 | 20.8 | 8.9 |
| 30 mpg | 9.4 | 25.0 | 10.6 |
| 35 mpg | 8.1 | 29.1 | 12.4 |
| 40 mpg | 7.1 | 33.3 | 14.2 |
| 45 mpg | 6.3 | 37.5 | 15.9 |
| 50 mpg | 5.6 | 41.6 | 17.7 |
| 55 mpg | 5.1 | 45.8 | 19.5 |
| 60 mpg | 4.7 | 50.0 | 21.2 |
| 65 mpg | 4.3 | 54.1 | 23.0 |
| 70 mpg | 4.0 | 58.3 | 24.8 |
How we calculate this
A UK (imperial) gallon is 4.54609 litres and a US gallon is 3.78541 litres, so the same car gets a higher mpg figure in UK gallons. A mile is 1.609344 km.
Litres per 100 km works the other way round to mpg: a lower number means a more economical car.
Questions people ask
Why are UK and US mpg different?
Because the gallons are different sizes. A UK gallon is about 20% bigger than a US gallon, so a car that does 40 mpg in the UK does about 33 mpg in US figures.
How do I convert mpg to litres per 100 km?
Divide 282.48 by the UK mpg. So 40 mpg is about 7.1 litres per 100 km. For US mpg, divide 235.2 instead.
What is good fuel economy?
For a modern petrol car, around 45 to 55 mpg in real driving is good. Diesels and hybrids often do better. Short trips and winter driving reduce it.
Related calculators
- Journey fuel cost calculatorWhat a car journey costs in fuel, split between passengers if you like.
- Electric vs petrol car running costYearly fuel cost of an electric car against petrol or diesel, at your mileage.
- EV charging time calculatorHow long an electric car takes to charge, on a 3-pin plug, wallbox or rapid charger.
- Mileage allowance calculatorHMRC approved mileage rates for 2026 to 2027: what you can be paid tax-free, or claim back.
- Car tax calculatorHow much vehicle tax (VED) a car costs a year, by CO2 and age.
- Stopping distance calculatorHighway Code stopping distances at any speed, in dry, wet or icy conditions.
Save or share this calculator
Embed this calculator
Free to use on your website, blog or newsletter. Copy this code and paste it into your page.