Gas Mileage Calculator guide
Gas Mileage Calculator: Work Out Your Car's MPG
Use our gas mileage calculator to work out your car's real fuel economy in UK mpg, US mpg and litres per 100km, and your fuel cost per mile.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
Your car's official mpg figure is measured in a lab. Our Gas Mileage Calculator works out what you actually get on the road.
How to use the calculator (Gas Mileage Calculator)
- Fill your tank to the top and reset your trip counter.
- Drive as normal until you next need fuel.
- Fill up again, then enter the distance on the trip counter and the fuel you put in.
- Add the fuel price to see your cost per mile.
The formulas
- UK mpg = miles ÷ (litres ÷ 4.546)
- Litres per 100km = litres ÷ km × 100
- Cost per mile = fuel cost ÷ miles
UK and US gallons
A UK (imperial) gallon is 4.546 litres, but a US gallon is only 3.785 litres. So the same car shows a higher figure in UK mpg than US mpg. Make sure you compare like with like.
A worked example
With the calculator's default settings, 350 miles on 35 litres of fuel is 45.5 UK mpg, or 6.2 litres per 100km. At 135.9p a litre, that's about 13.6p a mile.
How to improve your mpg
- Keep your tyres at the right pressure.
- Drive smoothly, and ease off the accelerator early.
- Take off roof boxes and racks when you're not using them.
- Avoid high speeds: 70 mph uses much more fuel than 60 mph.
Frequently asked questions
Why is my mpg lower than the official figure?
Real driving includes traffic, hills, cold starts and air conditioning, which the lab test doesn't fully reflect.
What's a good mpg?
It depends on the car. Many small petrol cars get 45 to 55 mpg, and many SUVs 30 to 40.
To work out the cost of a journey, use our Fuel Cost Calculator.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.