How to calculate your real MPG from fill-ups
Real MPG comes from the full-tank method. Fill the tank and note the odometer, then at your next full fill-up note the gallons you added and the new odometer reading. MPG is the difference between the two odometer readings divided by the gallons from the second fill-up. Dashboard MPG is an estimate and is commonly 5 to 10 percent off.
What you need
- Fill to full each time (stopping at the first pump click is the most consistent)
- The odometer reading right after filling
- Gallons and price, both on your receipt
The formula
MPG = (this odometer − last odometer) ÷ gallons this fill-up. With a last reading of 41,200 miles, a new reading of 41,530 miles and 10.5 gallons added: 330 ÷ 10.5 = 31.4 MPG.
For L/100km, use 235.2 ÷ MPG (US gallons); 31.4 MPG is about 7.5 L/100km. Cost per mile is the fill-up total divided by the interval distance.
How to average it
Averaging each interval's MPG over-weights short intervals. The correct average is total miles ÷ total gallons, leaving out the fuel from your very first full tank.
Common mistakes
- Counting the first fill-up's gallons. It is only the starting point.
- Treating a partial fill-up like a full one. Add it to the next full-tank interval instead.
- Skipping an entry. Leave that interval out of the average.
- Judging from one tank. Weather, traffic and A/C swing MPG; look at three or more intervals.
Log it with Tankbook
In Tankbook you enter the odometer and any two of price, amount and total, and the third is calculated. Average MPG is total distance ÷ total fuel, in MPG, km/L or L/100km per vehicle. It is free and needs no login.
Two full tanks and your real MPG appears
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