How to calculate real MPG with the full-tank method
The average MPG on your dashboard tends to read a little better than reality. The most accurate way to know your car's real fuel economy is the full-tank (tank-to-tank) method. All you need is your pump receipts and odometer.
What the full-tank method is
You divide the distance driven between two full fill-ups by the fuel you put in at the second one. Because the tank is full both times, the fuel you added equals the fuel you burned.
- Fill the tank to the click and note the odometer.
- Drive as usual.
- Fill up to full again and note the gallons and odometer.
- MPG = (current odometer − previous odometer) ÷ gallons added
Worked example
If there was a partial fill-up in between, add up all the fuel since the last full tank.
| Entry | Odometer | Gallons | Type |
|---|---|---|---|
| 1 | 32,000 mi | — | Full |
| 2 | 32,180 mi | 5.0 | Partial |
| 3 | 32,390 mi | 6.1 | Full |
| Result | 390 mi | 11.1 gal | 35.1 MPG |
When MPG is missing or looks wrong
With only one full-tank entry there's no interval to measure, so no MPG yet. Your first number appears after the second full fill-up.
A skipped fill-up inflates your MPG: the miles count but the fuel doesn't. Add the missing entry later with a past date.
L/100km is liters per 100 km; convert with 235.2 ÷ MPG (US).
Try it with Fillnote
Fillnote marks each entry as full or partial and calculates real economy between full tanks automatically. Type any two of cost, price and volume, pick MPG (US/UK), L/100km or km/L per vehicle, and see cost per mile on the home screen.
Start tracking real MPG at your next fill-up
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