MPG calculator
Two numbers from your last fill-up give you your car’s real fuel economy — in US MPG, imperial MPG and litres per 100 km at the same time, because those three names get used interchangeably and only one of them is yours.
About average for a mid-size petrol car.
Fill the tank, reset the trip meter, drive, then fill it again — the second fill is the fuel the trip used. One tank measures that tank: motorway driving will read far better than a week of short cold trips in the same car. Turn this into a cost per trip →
Turning MPG into money
Fuel economy only becomes useful next to a price. At today’s US average of $4.07/gal, a car doing 32 MPG costs about 13¢ a mile in fuel — around $1,536.91 over 12,000 miles a year. Divide your own MPG into the pump price to get your cost per mile, or use the tools below to do it against live prices in any of our markets.
Common questions
How do I calculate MPG?
Divide the miles you drove by the gallons it took to refill the tank. Fill the tank completely, note the odometer or reset the trip meter, drive normally, then fill it completely again — the amount that goes in on the second fill is exactly the fuel the trip used. 320 miles on 12 gallons is 320 ÷ 12, or 26.7 MPG.
Why is US MPG different from UK MPG?
Because the gallons are different sizes. An imperial gallon is 4.546 litres against the US gallon's 3.785 — about 20% larger — so the same car reads roughly 20% higher in UK MPG than US MPG without being any more economical. A car doing 30 MPG in the US does 36 MPG in the UK. This calculator shows both, plus L/100 km, so the number is never ambiguous.
Is a lower L/100 km better or worse?
Lower is better. L/100 km measures fuel consumed over a fixed distance, so it runs the opposite way to MPG, which measures distance covered on a fixed amount of fuel. 5 L/100 km is economical; 12 L/100 km is thirsty.
Why is my real MPG worse than the manufacturer's figure?
Official figures come from a standardised laboratory cycle, not from your commute. Short trips where the engine never reaches temperature are the single biggest cause of a gap, followed by cold weather, roof boxes, under-inflated tyres, and speed — drag rises with the square of velocity, so motorway cruising at 80 mph burns markedly more than at 65. A gap of 10–20% against the official number is ordinary.
How many miles per gallon does an average car do?
Across the 313 petrol models in our database the median is 7.4 L/100 km, or about 32 MPG (US) — roughly 38 MPG imperial. Small hatchbacks sit well above that and large SUVs and pickups well below.