How to calculate MPG accurately with partial fill-ups
You can calculate accurate MPG even when you don't always fill the tank. Treat fuel economy as one interval from a full tank to the next full tank, add up every gallon you put in during that interval, partial fill-ups included, and divide the miles driven by that total. Calculating a partial fill-up on its own gives wildly wrong numbers because you don't know how much fuel was left in the tank.
Why partial fill-ups break MPG
The fill-up method assumes the fuel you add equals the fuel you used since last time. That is only true when you fill to full. Add 5 gallons, then fill up next time, and the next fill-up carries two intervals' worth of fuel, so that interval's MPG drops by half.
The method: group by full tank to full tank
Between full fill-up A and the next full fill-up B, no matter how many partial fill-ups happened:
- Interval distance = odometer at B − odometer at A
- Interval fuel = every gallon added after A (partials plus B)
- MPG = interval distance ÷ interval fuel
Worked example
Fill-up A's gallons are not counted. That fuel belongs to the interval before A.
| Fill-up | Odometer | Gallons | Full? |
|---|---|---|---|
| A | 30,000 mi | 11.0 | Full |
| Partial | 30,180 mi | 4.0 | Partial |
| B | 30,372 mi | 7.5 | Full |
372 miles ÷ 11.5 gallons (4.0 + 7.5) = about 32.3 MPG. Calculated separately, you would see 45 MPG and then 25.6 MPG, neither of which is real.
If you missed a fill-up
A missing entry means the interval's fuel total is too small, so MPG comes out too high. The honest fix is to leave that interval out of the average and start a fresh interval at the next full tank.
Let Tankbook do it
Tankbook has a full/partial toggle and a "missed a fill-up" checkbox on every entry, and calculates MPG only from full tank to full tank. Partial fuel is added to the next interval automatically and missed intervals are left out. It is free and needs no login.
MPG that partial fill-ups can't break
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