This calculator measures rather than predicts. The method is brim to brim: fill the tank until the pump clicks off, note the odometer, drive normally, then fill to the click again at the same pump and note both the new odometer reading and the litres dispensed. Those litres are exactly the fuel the distance consumed, which is what makes the result trustworthy in a way a trip computer is not.
Miles per gallon is distance divided by fuel, so the only conversion needed is litres to gallons. One imperial gallon is 4.54609 litres and one US gallon is 3.785411784 litres, a difference of about 20 per cent — which is why the same car appears to do 50 mpg in Britain and 42 mpg in America. If your odometer reads in kilometres, divide by 1.609344 to get miles first.
A fill at 24680 miles and a refill at 25012 miles is 332 miles. Putting 34.5 litres in means 7.589 imperial gallons, so the car achieved 43.75 mpg — equivalently 6.46 litres per 100 km or 9.62 miles per litre. One tank is a rough guide; three consecutive tanks averaged is a genuine figure.
The formula
miles- Odometer difference between the two fills
litres- Fuel dispensed at the second fill
4.54609- Litres in one imperial gallon
3.785411784- Litres in one US gallon, if you are measuring in US units
How to use this calculator
- Fill the tank to the first automatic cut-off and note the odometer.
- Drive as you normally would — a mixed tank gives a more honest number than one motorway run.
- Refill to the cut-off at the same pump and enter the litres dispensed.
- Enter both odometer readings; the gauge shows mpg imperial and the metric equivalents sit below.
Worked examples
332 miles on 34.5 litres
34.5 litres is 34.5 ÷ 4.54609 = 7.589 imperial gallons. 332 ÷ 7.589 = 43.75 mpg. The same numbers give 6.46 litres per 100 km and 52.54 mpg in US units.
A kilometre odometer
A car showing 534 km between fills on 34.5 litres has covered 331.8 miles, giving 43.72 mpg imperial and 6.46 L/100 km. Always convert the distance before dividing, not after.
How to read your score
Frequently asked questions
Why does this disagree with my trip computer?
On-board computers infer consumption from injector open time and a fuel-flow model rather than measuring dispensed volume. They commonly read 3 to 10 per cent optimistic and rarely account for fuel used while idling in traffic in the same way. The brim-to-brim figure is the one to trust.
How many tanks should I measure?
At least three, then average them. A single tank is heavily influenced by weather, traffic, tyre pressures and how full the pump filled it. Averaging three consecutive tanks smooths most of that out.
Does it matter which pump I use?
Yes, more than you would think. Automatic cut-off points differ between nozzles and forecourts, and a sloping forecourt changes how much fuel the neck holds. Using the same pump at both ends removes most of that error.
Is it valid to fill only partly?
No. The method depends on the tank being at the same level at both readings, which only the automatic cut-off reliably reproduces. A partial fill breaks the assumption and the result is meaningless.
What about a plug-in hybrid?
Fuel-only mpg becomes close to meaningless once charging is involved, because the distance includes electric miles the fuel did not provide. Measure a tank with the battery deliberately depleted to get the engine-only figure, and account for electricity separately.
Fuel economy conversion reference
| Litres added | Imperial gallons | mpg (imp) | mpg (US) | L/100km |
|---|---|---|---|---|
| 25 L | 5.499 | 60.37 | 50.27 | 4.68 |
| 30 L | 6.599 | 50.31 | 41.89 | 5.61 |
| 35 L | 7.699 | 43.12 | 35.91 | 6.55 |
| 40 L | 8.799 | 37.73 | 31.42 | 7.49 |
| 45 L | 9.899 | 33.54 | 27.93 | 8.42 |
| 50 L | 10.998 | 30.19 | 25.14 | 9.36 |
| 60 L | 13.198 | 25.16 | 20.95 | 11.23 |
Distance held at 332 miles. Every figure is derived from 4.54609 litres per imperial gallon and 1.609344 km per mile.