How to use the MPG Calculator
- Fill the tank completely and note the odometer or reset the trip meter.
- Drive normally until the tank is low, then fill completely again.
- Enter the miles covered and the fuel added at the second fill.
- Repeat over three or four tanks and average the results.
How the calculation works
MPG is miles driven divided by gallons added, where the fuel added at the second fill is exactly what the measured distance consumed — provided both fills go to the same level. That is why brim-to-brim measurement matters: a partial fill breaks the correspondence between distance and fuel, and a single tank's figure also reflects whatever mix of driving that tank happened to contain.
Metric consumption inverts the relationship: litres per 100 km falls as efficiency rises, and converts as L/100km = 235.215 ÷ MPG for US gallons, or 282.481 ÷ MPG for imperial. Note which gallon a figure uses, since a US gallon is 3.785 litres against the imperial 4.546 — a 20% difference that makes UK and US MPG figures look better or worse than they are. On-board computers typically read optimistically by a few percent, so a measured average is the better number.
MPG = miles / gallons added ; L/100km = litres / (km / 100) ; L/100km = 235.215 / US MPGSource: US gallon 3.785412 L and imperial gallon 4.546 L definitions; conversion constants derived from those.
Worked example
Brim-to-brim over three tanks: 348 miles on 11.2 gal, 402 on 13.6 gal, 291 on 9.4 gal.
- Tank 1: 348 / 11.2 = 31.1 MPG.
- Tank 2: 402 / 13.6 = 29.6 MPG.
- Tank 3: 291 / 9.4 = 31.0 MPG.
- Combined: 1,041 miles / 34.2 gal = 30.4 MPG.
30.4 MPG combined, equivalent to 7.7 L/100km — the total-miles-over-total-gallons figure, which is the correct way to combine tanks.
Frequently asked questions
Why must I fill the tank completely?+
Brim-to-brim filling ensures the fuel added equals what the measured distance used. A partial fill leaves the start and end levels different, so the figure is meaningless.
Should I average the per-tank MPG figures?+
No — divide total miles by total gallons. Averaging the individual figures weights short tanks equally with long ones and gives a slightly wrong answer.
Is my trip computer accurate?+
It is usually optimistic by a few percent because it estimates from injector timing rather than measuring fuel. A brim-to-brim average over several tanks is more trustworthy.
Why is a UK MPG figure higher than a US one for the same car?+
The imperial gallon is 4.546 litres against the US 3.785, so the same efficiency yields a roughly 20% higher imperial number. Always check which gallon a figure uses.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.