The MPG formula
MPG = miles driven ÷ gallons used · L/100 km = litres used ÷ km driven × 100
Worked examples
- 320 miles on 11.5 US gallons = 27.8 MPG (8.45 L/100 km).
- UK: 410 miles on 42 litres. 42 L = 9.24 imperial gallons, so 410 ÷ 9.24 = 44.4 MPG.
- 620 km on 41.5 litres = 6.69 L/100 km (14.9 km/L).
Getting an accurate figure
- Use the same pump, and let it click off once — do not top up.
- Average over two or three tanks; one tank can be skewed by traffic or weather.
- Winter fuel blends, cold starts and short trips all lower MPG noticeably.
Compare your result with the official figures on our US fuel economy pages, or convert it with the fuel economy converter.
Frequently asked questions
How do I measure my real MPG?
Fill the tank until the pump clicks off and reset the trip meter. Drive normally, then fill up again the same way. Divide the miles on the trip meter by the gallons you just added.
Why is a UK gallon different from a US gallon?
The imperial (UK) gallon is 4.546 litres and the US gallon is 3.785 litres. The same car therefore shows about 20 % more MPG in UK units than in US units.
Why does my trip computer show better MPG than my calculation?
Trip computers estimate fuel from injector timing and are often a few percent optimistic. The fill-up method measures what actually went into the tank, so it is the more reliable figure.
Can tire size make my MPG reading wrong?
Yes. If your tires are taller or shorter than standard, the odometer distance is off by the same percentage, and so is your MPG. Use the tire size and fuel economy calculator to correct it.