Quick answer
- Fill the tank and record the odometer.
- At the next full fill, subtract odometer readings.
- Divide miles driven by gallons added.
- For several tanks, divide total miles by total gallons—do not average MPG values.
The full-tank MPG formula
Fuel economy is distance divided by fuel. The tricky part is knowing how much fuel was actually consumed. A fuel gauge is not precise enough, so the standard hand-calculation uses two fills at the same full point.
The fuel added at the second fill replaces what the vehicle used since the first. That is why the first entry establishes a baseline but cannot produce an MPG result by itself.
Step-by-step example
You fill at 42,000.0 miles. At the next fill the odometer reads 42,327.4, and the pump adds 11.62 gallons.
MPG = 327.4 ÷ 11.62 = 28.18 MPG
Record at least one decimal place for distance and two or three for volume. More display digits do not make the measurement perfect, but premature rounding adds avoidable error.
How to calculate L/100km
Liters per 100 kilometers measures fuel used for a fixed distance, so lower is better. Use the same full-to-full process, but divide liters by kilometers and multiply by 100.
For 41.2 liters across 535 kilometers, the result is 41.2 ÷ 535 × 100 = 7.70 L/100km. The direct US conversion is L/100km = 235.215 ÷ MPG.
Partial fills are usable
A partial fill does not identify the fuel consumed since the previous fill, because the tank did not return to a known endpoint. It can still be included if the measurement begins at full and eventually ends at full. Add every partial purchase plus the fuel at the ending full fill, then divide the complete distance by that sum.
Start full at 10,000 miles. Add 4.0 gallons at 10,120 without filling. At 10,250, add 6.0 gallons and fill the tank.
Use a weighted average across tanks
An ordinary average gives every tank equal influence, even when one covered 100 miles and another covered 400. The correct fleet-style average combines the totals.
Suppose one segment is 100 miles on 5 gallons (20 MPG) and the next is 300 miles on 10 gallons (30 MPG). The ordinary average is 25 MPG, but the correct combined result is 400 ÷ 15 = 26.67 MPG.
Why one tank can be misleading
The pump may stop at a slightly different liquid level. The vehicle might sit at another angle. Temperature changes fuel volume. Remote start, idling, short trips, headwinds, traffic, roof racks, tire pressure, towing, and spirited driving all change consumption. A single tank is a data point; several complete tanks form a trend.
If MPG suddenly changes, first check whether the fill was comparable. Then review tire pressure and size, trip pattern, weather, fuel blend, maintenance, warning lights, and any recent modification. Fuel economy alone does not diagnose a failed component.
MPG and cost per mile answer different questions
MPG tells you fuel efficiency. Cost per mile includes pump price, which can make a more efficient premium-fuel vehicle cost more per mile than a regular-fuel vehicle. Calculate each fill's cost across the same full-to-full segment.
When comparing fuels such as gasoline and E85, use measured economy and actual price for each. Do not assume one fixed MPG penalty because ethanol content and vehicle response vary.
Make the measurement repeatable
- Use the same full-tank method each time; do not keep topping off after the nozzle clicks.
- Record the odometer, not a remembered estimate.
- Log partial fills instead of discarding them.
- Check that tire-size changes have not altered odometer calibration.
- Compare several tanks before calling a small change meaningful.
- Export a backup if your tracker stores data only in the browser.
Track it without an account
The TunerBench MPG Calculator and Fuel Log implements the full-to-full method, carries partial fills forward, calculates a distance-weighted average, and exports CSV. Entries stay in your browser, so keep an export if the history matters.