How to Measure Fuel Economy with a Fuel Log
A useful fuel log matches distance traveled with the fuel consumed over the same interval. Start and finish at comparable full fills, and keep every fuel receipt in between.
By AutomotiveCalc · Published September 17, 2026
Already have totals? Enter them in the MPG Calculator. This guide explains which receipts belong in those totals, how to handle partial refills, and how to compare several tanks without averaging them incorrectly.
Start a full-to-full log
- Fill according to your vehicle manual and record the odometer and date. This establishes the starting fuel level. Do not include this first purchase in the fuel consumed during the new log.
- At each later purchase, record the odometer, fuel quantity, unit, and whether the fill was full or partial. Keep every receipt, including small purchases.
- Finish at another comparable full fill. Subtract the starting odometer from the final reading. Add all fuel purchased after the starting fill, including the final fill.
Use a consistent filling method and stop at the first automatic shutoff; do not top off to force an exact volume. Ford's fuel consumption guidance describes consistent filling and a log spanning several fill-ups. Follow your own vehicle's manual for refueling instructions.
A partial-refill example
These invented readings use miles and U.S. gallons. The initial purchase establishes the boundary; it is not fuel used after that boundary.
| Odometer | Purchase | Count it? |
|---|---|---|
| 10,000 mi | Full fill: 9 gal | No: starting point |
| 10,200 mi | Partial: 5 gal | Yes |
| 10,400 mi | Full fill: 8 gal | Yes: final fill |
Distance = 10,400 − 10,000 = 400 miles. Counted fuel = 5 + 8 = 13 U.S. gallons. Fuel economy = 400 ÷ 13 = 30.77 U.S. mpg. Do not divide the first 200 miles by the partial purchase: the fuel level at that stop does not match the initial full level.
For your own log, keep columns for date, odometer, fuel volume, fuel unit, full/partial status, and notes such as towing or unusual idling. A missing fuel quantity makes that interval incomplete. Start a fresh interval at a known full fill instead of silently treating the missing purchase as zero.
How to average MPG across several tanks
Combined MPG = total miles ÷ total gallons. Do not normally take the simple average of the MPG readings. For example, one complete interval of 300 miles on 10 gallons is 30 mpg. Another of 200 miles on 5 gallons is 40 mpg. Together they give 500 ÷ 15 = 33.33 mpg, not (30 + 40) ÷ 2 = 35 mpg.
MPG is weighted by fuel volume: (30 × 10 + 40 × 5) ÷ (10 + 5). A simple average works when the fuel quantities are equal. Adding the original distance and fuel records is usually easier and avoids rounding each tank first.
For metric records, combined L/100 km = 100 × total liters ÷ total kilometers. A 400 km interval using 32 L and a 100 km interval using 6 L combine to 38 L over 500 km, or 7.6 L/100 km. The simple average of 8 and 6 would be 7, which is incorrect here. L/100 km is weighted by distance.
Never add miles to kilometers or U.S. gallons to Imperial gallons. Convert records to a common basis first. The fuel economy converter distinguishes the two gallon definitions; a quoted MPG figure without its gallon convention is ambiguous.
Why one tank can look unusually good or bad
The method assumes the fuel quantity in the tank is the same at both endpoints. If the final fill leaves more fuel in the tank, the pump quantity overstates fuel burned and calculated MPG falls. If it leaves less, calculated MPG rises. Tank inventory obeys: fuel burned = fuel purchased + starting fuel − ending fuel.
To illustrate sensitivity, suppose a car actually burns 10 gallons over 300 miles. If endpoint variation makes the recorded refill 10.5 gallons, the estimate is 28.57 mpg; at 9.5 gallons it is 31.58 mpg. These are hypothetical differences, not a claimed pump tolerance. Over 900 miles and 30 gallons, the same half-gallon endpoint difference gives about 29.51 or 30.51 mpg.
A longer continuous log reduces the relative effect of an endpoint mismatch. It does not repair missing receipts, an incorrect distance reading, or mixed units. Intermediate fill-level differences cancel in a continuous total when all purchases are included; only the starting and ending inventory remain in the balance.
Compare like driving with like driving
Keep trip type and conditions alongside the numbers. A change in the log is a measured change for that recorded interval, not proof of a mechanical fault or the benefit of a modification. Compare several complete intervals with similar use before drawing a conclusion. Use your measured average in the Fuel Cost Calculator or Fuel Range Calculator, allowing for conditions that may differ on the next trip.
Common questions
Can I use the fuel gauge instead of refilling?
A gauge position does not provide a measured fuel volume. This method uses pump quantities between comparable full fills, rather than assigning gallons to gauge marks.
Must I calculate each partial refill separately?
No. Include its fuel in a complete full-to-full interval. You cannot assign a reliable separate MPG to that partial segment from its purchase alone.
Does a dashboard MPG difference mean something is broken?
This log alone cannot diagnose the cause. First check matching reset intervals, units, complete receipts and comparable fill levels. Consult the vehicle manual for how its display defines average consumption.
Sources and method
The Ford manual linked above supports the consistent-refill and recorded-distance method (accessed September 17, 2026). The weighting and inventory examples are original arithmetic derivations, not vehicle test results. Automated checks verify the worked numbers. This guide has not received an independent automotive professional review. See our editorial standards or report an error.