How the MPG calculation works
Fuel is sold in litres but UK MPG is based on the imperial gallon, so the calculator converts first:
UK MPG = miles ÷ (litres ÷ 4.546)
Driving 350 miles on 40 litres means you used 8.8 gallons — about 39.8 MPG.
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.
Is your trip computer right?
Many cars' trip computers read a little optimistic. Checking now and then with the brim-to-brim method gives you a reliable figure to use in the fuel cost calculator.
What affects MPG?
- Speed — fuel use rises noticeably at higher motorway speeds.
- Short journeys and cold engines.
- Tyre pressures, weight and roof racks.
- Stop-start traffic and hilly routes.