Formula
Imperial MPG = miles ÷ (litres ÷ 4.54609)
Fill-to-fill method
Fill the tank, reset the trip counter, drive normally, then refill. The litres added at the second fill approximate what was used over those miles.
Why MPG changes
Traffic, speed, weather, tyres, load and driving style can all affect real-world fuel economy.
Worked example
400 miles using 40 litres works out at about 45.5 MPG imperial.