Where to get the figures
Fill the tank until the pump first clicks off and reset the trip meter. Next time you fill up, stop at the first click again: the fuel on the receipt and the miles on the trip meter are the figures you need. Averaging several tanks gives a more reliable result. Step by step, with an example: how to work out your car’s real mpg.
In the UK fuel is sold by the litre, so choose “UK” and enter litres and miles: the calculator converts to imperial gallons. In the US, enter gallons. Your trip computer is a useful guide but not always right: Germany’s ADAC checked 80 models against measured fuel use and only 25 were within 2%. Most, in fact, showed higher consumption than the car really used.
Use your result to cost a trip
With your real mpg you can work out the cost of any journey in the trip fuel cost calculator: enter the result and choose “My real figure” under “Where does this figure come from?”. Because it is your actual fuel economy, we don’t correct it as we do official figures. It also works for your commuting costs.
Frequently asked questions
Why are UK and US mpg different?
Because the gallons are different: an imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres. For the same fuel use, the UK mpg figure is 20% higher.
How do I convert mpg to L/100 km?
Divide 282.48 by UK mpg, or 235.21 by US mpg. For example, 47 mpg (UK) is 6.0 L/100 km. The calculator shows all the figures at once.
Why does it say my result is unusual?
If it works out below 2 or above 30 L/100 km (above about 140 or below about 9 mpg UK), check your figures: usually the fuel wasn’t a full fill-up or the miles are from a different stretch. It still shows the result.