Different fill level
Stopping at a different point in the filler neck changes the litres recorded even if the vehicle used the same fuel.
How to calculate your real MPG properly
Dashboard MPG is convenient, but a measured full-to-full calculation gives you a repeatable figure from fuel actually added and miles actually driven.
Reviewed 6 September 2026UK units unless statedCalculations and assumptions shown
The calculation in one line
UK MPG = miles travelled ÷ (litres used ÷ 4.54609)
The key point is the gallon: MotoringCalcs uses the UK imperial gallon, which is 4.54609 litres. A US MPG figure is not directly comparable because the US gallon is smaller.
Full-to-full method
- Fill the tank to a consistent stopping point and reset the trip counter.
- Drive normally. A longer interval is usually more representative than a short test loop.
- At the next fill, record the trip miles and the litres needed to return the tank to the same stopping point.
- Divide litres by 4.54609 to get imperial gallons, then divide miles by gallons.
You do not need to know the tank capacity. The relevant fuel quantity is the amount used between the two matched fills.
Worked example: 412 miles and 39.8 litres
1
39.8 L ÷ 4.54609 = 8.754 imperial gal
2
412 miles ÷ 8.754 = 47.06 UK MPG
For planning, using about 47 MPG is more defensible than copying a brochure figure if this measurement reflects your normal roads and driving.
A simple three-fill log is better than one unusually good tank
| Fill | Miles | Litres | Measured UK MPG |
|---|---|---|---|
| 1 | 412 | 39.8 | 47.1 |
| 2 | 386 | 38.1 | 46.1 |
| 3 | 438 | 40.6 | 49.0 |
| Combined | 1,236 | 118.5 | 47.4 |
Combining the totals avoids over-weighting a small fill and smooths out some pump shut-off variation.
What can distort a tank-to-tank result?
Short distance
A small error in litres has a larger effect when the distance between fills is short.
Different use
Cold starts, traffic, towing, roof boxes, motorway speed and weather can make one tank unrepresentative of your usual driving.
Which MPG should you use in another calculator?
Use a recent average that matches the decision. A commute calculator should use economy typical of that commute; a towing budget should use towing economy; an annual fuel-cost estimate should use a broader average. The aim is not to find the highest MPG your car has achieved, but the figure that best represents the miles you are trying to cost.