Advanced options
Uses the UK gallon (4.546 litres). The most accurate method is brim-to-brim: fill the tank, drive normally, refill to the brim and enter the miles covered and litres it took.
What The MPG Calculator Works Out
- Miles per gallon (UK): miles driven divided by gallons used, where a UK gallon is 4.546 litres, about 20% bigger than the US gallon. That is why UK mpg figures always look better than American ones for the same car. The gold-standard measurement is brim-to-brim: fill the tank, zero the trip, drive normally, refill to the brim and the pump shows exactly what you used.
- Litres per 100km: the metric convention most of Europe uses, where lower is better. 40mpg is about 7.1 L/100km, 50mpg about 5.6 and 60mpg about 4.7.
- Miles per litre: miles divided by litres, handy for quick mental sums at the pump. 8 miles per litre is about 36mpg.
- Units entered in (advanced): switch the distance to kilometres (for a trip computer or hire car set to metric) or the fuel to UK gallons. Everything is converted to miles and litres before the sums run.
- Fuel type and price (advanced): picking diesel loads the current average diesel price; override either with the price on your receipt. Supermarket forecourts are usually cheapest and motorway services run 15p to 20p a litre dearer.
- Fuel cost per mile and this tank cost (advanced): litres multiplied by the pump price, then divided by the miles covered. Most petrol and diesel cars sit between 12p and 25p a mile for fuel alone at current prices.
- Annual fuel cost (advanced): your yearly mileage divided by the measured mpg gives litres a year, multiplied by the price. The average UK car covers around 7,000 to 8,000 miles a year.
- Versus the official figure (advanced): your measured mpg as a percentage of the manufacturer's WLTP combined figure. Expect to undercut it by 10% to 20%; the lab test is gentler than real traffic. A sudden drop with no change in driving usually points to tyre pressures, a sticking brake caliper, short winter journeys or a tired sensor.
- CO2 per mile (advanced): litres multiplied by 2.31 kg for petrol or 2.68 kg for diesel (DEFRA factors), spread over the miles, with the g/km figure that road tax and company car bands use alongside.
- US mpg equivalent (advanced): the same economy in US gallons (3.785 litres), useful when reading American reviews or forums: 40 UK mpg is only 33 US mpg.
Source and further reading: RAC Fuel Watch.