The brim-to-brim method
Trip computers can be optimistic. For a reliable figure:
- Fill the tank until the pump clicks off, and reset your trip meter.
- Drive as you normally would — ideally a full tank's worth.
- Fill up again at the same pump if you can, and note the litres.
- Enter the miles from the trip meter and the litres into the calculator.
Example
400 miles on 45 litres works out at about 40.4 UK MPG, or 7.0 L/100km.
MPG vs L/100km
MPG tells you how far you go on a fixed amount of fuel (higher is better). L/100km tells you how much fuel a fixed distance takes (lower is better). Both describe the same thing; the UK mostly uses MPG, while much of Europe uses L/100km.
What is UK MPG?
UK MPG measures how many miles a car travels on one imperial gallon of fuel — 4.546 litres. A US gallon is smaller, at 3.785 litres, so the same car shows a lower MPG figure in US units. A car returning 50 MPG in the UK would read about 41.6 MPG in US terms.
That matters if you're using figures from American websites or reviews: putting a US MPG number into a UK calculator will overstate your fuel cost. Everything on this site uses UK imperial MPG unless you switch to L/100km.
Now you know your real MPG, use it in the fuel cost calculator for accurate journey costs.