GarageCalc 
The fill-up method, step by step
- Fill the tank until the pump clicks off. Reset the trip odometer.
- Drive normally until the tank is at least half empty (longer stretch = more accurate).
- Fill up at the same pump speed until it clicks off again.
- Enter the trip miles and the gallons from the receipt above.
One tank gives a good estimate; averaging three or four tanks gives a number you can trust for budgeting. For trip planning with that number, use the fuel cost calculator.
How it's calculated
Changed tire size? Your MPG math is skewed
The odometer counts wheel revolutions calibrated to the factory tire. Taller tires make it under-count miles, so your calculated MPG reads worse than reality; shorter tires do the opposite. The error equals the diameter change percentage: check yours with the tire size comparison calculator and correct the miles before dividing.
Frequently asked questions
How do I calculate my real MPG?
Fill the tank completely and reset the trip odometer (or note the mileage). Drive normally, fill the tank completely again, and divide the miles driven by the gallons it took to refill. That is your true fuel economy for that stretch of driving, more honest than the dashboard display.
Why does my dashboard MPG differ from the calculated one?
Trip computers estimate fuel flow rather than measuring pump gallons, and most read 3 to 8 percent optimistic. The fill-up method also catches things the computer misses, like fuel lost to idling with the ignition on. If you changed tire size, the odometer itself reads wrong too, which skews both numbers.
What is a good MPG?
As of 2026 in the US: hybrids typically return 45-55 MPG, efficient compacts 32-40, mid-size sedans 28-35, crossovers 25-32, full-size trucks 17-22, and heavy-duty or performance vehicles under 17. Diesel trucks do better under load than their gas MPG twins suggest.
How do I convert MPG to liters per 100 km?
Divide 235.215 by the MPG figure. For example, 28 MPG equals 8.4 L/100km. This calculator shows both automatically, plus imperial MPG used in the UK.