Miles Per Gallon Calculator

Miles per gallon measures how far your car travels on one UK gallon of fuel, which is 4.546 litres. Higher MPG means lower running costs, and the only way to know your real figure is to measure it. Fill the tank to the brim, reset the trip counter, drive normally for a tankful, then fill up again and note both the litres added and the distance covered. Dashboard readouts are convenient but tend to flatter the car by 5 to 15%, so a brim-to-brim measurement is worth doing at least once.

The calculation itself is distance in miles divided by fuel used in UK gallons. On a 350-mile trip that used 40 litres, dividing by 4.546 gives 8.80 gallons, and 350 divided by 8.80 is 39.8 MPG. At £1.72 a litre, the UK average petrol price in the week commencing 21 September 2026 (DESNZ), that tankful cost £68.80, or 19.7p a mile, and the calculator lets you enter the price you actually paid. Watch out for American figures quoted online, because a US gallon is only 3.785 litres, so the same car always looks thirstier in US units. Multiply a UK figure by 0.833 to get the US one, or a US figure by 1.201 to come the other way: a Toyota Prius rated at 64 MPG in the UK is 53 MPG in the United States. The calculator also converts your result into litres per 100km and kilometres per litre.

Official WLTP manufacturer figures typically exceed real-world MPG by 10 to 25%, which is why a car advertised at 60 MPG realistically returns 50 to 54 in mixed driving. WLTP has applied to new UK cars since 2017 and is closer to reality than the old NEDC cycle it replaced, where models sold between 2010 and 2017 often undershot their quoted figures by 25 to 30%. Independent measurements from What Car? True MPG, Honest John Real MPG and Autocar give a better guide than the brochure. Electric range is quoted on the same optimistic basis, and cold weather takes a further 20 to 30% off it.

What you get in practice depends heavily on the type of car. Small petrol models such as a Fiesta or Polo return 50 to 65 MPG combined, family petrol cars like a Focus or Golf 40 to 55, and petrol SUVs such as a Qashqai or Sportage 35 to 45. A family diesel manages 50 to 65, and full hybrids like a Prius or Yaris reach 60 to 80. Plug-in hybrids show more than 100 MPG while the battery lasts, then fall back to ordinary petrol economy after 30 to 50 miles. Sports cars such as an M3 or RS6 sit at 25 to 35. An electric car returning 4 to 5 miles per kWh is equivalent to roughly 120 to 150 MPGe.

Driving style moves the number as much as the badge does. A steady 50 to 60 mph is the efficient zone, and sustained speeds above 70 cost 10 to 20%. Hard acceleration and braking waste 30 to 40% of your fuel. Tyres under-inflated by half a bar cost 5 to 10%, a roof rack costs 10 to 25% at motorway speed, and the first five miles from cold waste 10 to 15% while the engine warms up. Air conditioning takes 5 to 10%, and cruise control on a flat motorway gives you 5 to 10% back.

Knowing your real MPG turns into a cost per mile, which is the figure that actually matters. A petrol car doing 45 MPG at £1.72 a litre (UK average, week commencing 21 September 2026) costs 17.4p a mile, an electric car charged at home on an 8p off-peak rate costs about 2p, and the same EV on public rapid chargers at 60 to 80p per kWh costs 15 to 20p, roughly what petrol costs. Over 8,000 miles a year that is around £1,390 of petrol against £150 to £300 of home electricity. HMRC's approved mileage rate for business travel is 55p per mile for the first 10,000 business miles and 25p thereafter for cars, raised from 45p on 6 April 2026, and it is meant to cover fuel, maintenance, insurance and tax rather than fuel alone.