How to work out your car's real MPG
The figure on your dashboard isn't always right. Here's how to measure your true fuel economy with one full tank.
Your fuel cost estimate is only as good as the MPG you enter. Official figures come from lab tests and trip computers can be optimistic, so measuring it yourself gives you a much more honest number.
The brim-to-brim method
- Fill the tank until the pump clicks off. Don't top up past the click.
- Reset your trip meter to zero.
- Drive normally until you need fuel again.
- Fill up at the same pump if you can, again stopping at the first click.
- Note the miles on the trip meter and the amount of fuel you just bought.
Do the maths
In the UK, pumps sell in litres but MPG uses imperial gallons. Divide the litres by 4.546 to get gallons, then divide the miles by that number.
Example: 380 miles on 38 litres. 38 ÷ 4.546 = 8.36 gallons. 380 ÷ 8.36 = about 45.5 mpg.
In the US, pumps sell in US gallons, so it's simply miles divided by gallons.
Example: 310 miles on 10.5 gallons = about 29.5 mpg.
Why UK and US MPG differ
A UK (imperial) gallon is about 20% bigger than a US gallon. The same car will show a higher MPG figure in the UK than in the US, even though it uses exactly the same fuel. Always use the figure for your own country in the calculator.
Get a better average
One tank tells you something, but three or four tanks across different kinds of driving tell you much more. Town driving, motorway runs and winter weather all change the result. Use your average for everyday planning, and your motorway figure for long trips.