Note: costs mentioned below are given in US dollars as a general point of reference — actual prices vary by country, currency, and local market.
Record the odometer, the volume pumped and the price at every fill-up. Miles since the last fill divided by the volume pumped is your real fuel economy — the only figure based on what actually left the pump rather than an estimate. Three or four consecutive fill-ups, always topped to the same point, gives a usable baseline. Expect it to sit below the official sticker or brochure figure, because those come from a controlled test cycle and your commute does not.
Why the trip computer and the pump disagree
A trip computer infers fuel used from injector open time. It is usually close, but the error is systematic rather than random, so it does not average out — most read slightly optimistic. The fill-to-fill method has no such problem: the pump measured the fuel, and the odometer measured the distance.
How to log a fill-up correctly
- Fill to the same point every time — the first automatic click of the pump handle. Topping off past that click is the main source of noise in the data.
- Write down the odometer reading before you drive away.
- Write down the volume pumped and the total paid.
- Work out miles since the last fill, then divide by the volume pumped. That is your figure for that tank.
Any notebook, notes app or spreadsheet will do. What matters is that the entries are consecutive and consistent — a single skipped fill-up breaks the chain for that tank.
Worked example
| Fill-up | Odometer | Gallons | Price/gal | Miles since last | MPG |
|---|---|---|---|---|---|
| 1 | 24,110 | 11.2 | $3.29 | — | — |
| 2 | 24,432 | 10.8 | $3.35 | 322 | 29.8 |
| 3 | 24,761 | 11.0 | $3.41 | 329 | 29.9 |
Two consecutive tanks agreeing to within a tenth of a mile per gallon is a solid real-world baseline for that car under that pattern of driving. Compare it against the official combined rating on the window sticker, which will usually be several MPG higher.
A partial fill, a different pump cut-off, or one atypical week of driving will throw a single entry out. Judge the average across several tanks and ignore the outlier.
What actually moves your figure
- Stop-start city traffic — the single biggest factor, and the reason a highway-rated figure is unreachable on an urban commute.
- Cold weather — longer warm-up periods, thicker oil and more accessory load all cost efficiency; a hard winter shows clearly in a fill-up log.
- Aerodynamic load — a roof box or a trailer costs far more than its weight suggests, because drag rises with the square of speed.
- Under-inflated tyres — measurably increases rolling resistance. Check pressures monthly against the placard in the driver's door shut, not the number on the tyre sidewall.
Turning the log into a budget
Once you have four to six weeks of entries, total the spend and divide by the period. That gives a monthly fuel figure grounded in your own driving — the number you need to compare honestly against a prospective EV's charging cost, a shorter commute, or a different car. Published averages are not a substitute, because fuel spend varies more between two drivers of the same car than it does between two different cars.
Frequently asked questions
How many fill-ups before the number is reliable?
Why is my real MPG lower than the official rating?
Does this work for diesel, hybrid and plug-in hybrid cars?
Should I use US or imperial gallons?
Can I use a fuel log for business mileage claims?
Sources & further reading
- US EPA — how fuel economy is tested
- US Department of Energy — driving habits and fuel economy
- GOV.UK — vehicle fuel consumption and emissions testing
Figures, prices and policy details were current at the last-updated date above. Automotive pricing, incentives and regulations change frequently — verify time-sensitive details with the linked primary sources. Read our editorial policy and fact-checking standards.
