How to work out your car's real MPG
Published - 4 min read
A simple, accurate way to measure your real miles per gallon, convert it to litres per 100km, and work out what each mile costs you.
Why measure it yourself?
The official economy figure for a new car comes from a standardised laboratory test (WLTP, used for new cars since 2018). It's useful for comparing models, but your real-world figure depends on your journeys, driving style, load and the weather. Trip computers are handy but can be optimistic. Measuring it yourself tells you what your car actually does.
The brim-to-brim method
- Fill the tank until the pump clicks off automatically. Don't top up after the click.
- Reset your trip meter (or note the odometer reading).
- Drive normally until you next need fuel.
- Fill up again until the first click, ideally at the same station and pump.
- Note the litres added and the miles driven since the last fill.
The litres you added on the second fill is the fuel you used over those miles.
The calculation
UK miles per gallon uses the imperial gallon, which is 4.54609 litres.
- MPG = miles driven / (litres used / 4.54609)
- Litres per 100km = 282.48 / MPG
| Example | Value |
|---|---|
| Miles driven | 412 |
| Litres used | 42.5 |
| Gallons used (42.5 / 4.54609) | 9.35 |
| MPG (412 / 9.35) | 44.1 |
| Litres per 100km (282.48 / 44.1) | 6.4 |
Beware American figures: a US gallon is smaller (about 3.785 litres), so US mpg numbers look lower for the same car.
What does each mile cost?
Once you know your MPG, you can work out your fuel cost per mile:
- Cost per mile (pence) = price per litre (pence) x 4.54609 / MPG
Or more simply, divide what you paid for the fill-up by the miles you drove on the previous tank. Use today's price at your usual station, or the national average on the UK Fuel Price Index.
Getting a reliable figure
- Repeat over several tanks and average the results - a single tank can be skewed by one long motorway run or a week of short trips.
- Use the same pump where you can, as the point where the nozzle clicks off can vary between pumps.
- Keep notes: date, odometer, litres and price. A simple spreadsheet or notes app is enough.