How to measure your mpg
- Fill the tank until the pump clicks off, and reset the trip meter.
- Drive as you normally do, ideally for at least half a tank.
- Fill up again until the pump clicks off, at the same pump if you can. Note the litres.
- Enter the miles from the trip meter and the litres. The result is your real-world mpg.
mpg = miles ÷ (litres ÷ 4.546)
For example, 350 miles on 35 litres is 350 ÷ 7.70 gallons = 45.5 mpg.
MPG to litres per 100km
Most of Europe measures fuel use as litres per 100km, where lower is better. To convert UK mpg, divide 282.48 by the mpg. To convert US mpg, divide 235.21 by it.
| UK mpg | L/100km | km per litre | US mpg |
|---|---|---|---|
| 20 | 14.1 | 7.1 | 16.7 |
| 25 | 11.3 | 8.9 | 20.8 |
| 30 | 9.4 | 10.6 | 25.0 |
| 35 | 8.1 | 12.4 | 29.1 |
| 40 | 7.1 | 14.2 | 33.3 |
| 45 | 6.3 | 15.9 | 37.5 |
| 50 | 5.6 | 17.7 | 41.6 |
| 55 | 5.1 | 19.5 | 45.8 |
| 60 | 4.7 | 21.2 | 50.0 |
| 65 | 4.3 | 23.0 | 54.1 |
| 70 | 4.0 | 24.8 | 58.3 |
Why the dashboard mpg is often wrong
The trip computer usually reads 5 to 10% higher than the real figure, because it estimates fuel flow rather than measuring what goes in the tank. The brim-to-brim method above is the most reliable way to check. Do it over several tanks to smooth out differences in how full the pump fills the tank.
What is good mpg?
For a modern petrol family hatchback, 45 to 55 mpg in mixed driving is good. Small diesels can reach 60 to 70 mpg on the motorway, and full hybrids often beat 60 mpg around town. Large SUVs and performance cars may manage 25 to 35 mpg.
Sources
- UK Weights and Measures Act 1985 (imperial gallon = 4.54609 litres)
- GOV.UK: Find out the fuel consumption of a car
Rates and rules last checked 15 October 2026. These figures are a guide. For a decision that matters, confirm with the official source.