Note: costs mentioned below are given in US dollars as a general point of reference — actual prices vary by country, currency, and local market.
The Fuel Expense Log records odometer, gallons/liters, and price at each fill-up to calculate your real-world MPG (or L/100km) and monthly fuel spend — figures that typically run 10-20% below EPA window-sticker estimates because of city driving, cargo, and weather. Logging just 3-4 consecutive fill-ups gives a usable efficiency baseline; a full month gives a reliable monthly cost figure.
At a glance
| Metric | Typical value |
|---|---|
| Real-world MPG vs sticker | 10-20% lower on average |
| Fill-ups needed for a baseline | 3-4 consecutive |
| Average US household fuel spend | $150-$220/month (1 vehicle) |
| Data needed per entry | Odometer, gallons, price/gal |
Why your dashboard MPG and your real MPG differ
Trip computers estimate fuel use from injector timing and are usually close, but small errors compound. The fill-to-fill method — dividing miles driven by gallons actually pumped — is the closest thing to ground truth, because it's based on what left the pump, not an estimate.
How to log a fill-up correctly
- Always fill to the same point (first click of the pump handle) for consistency.
- Record the odometer reading at each fill.
- Enter gallons (or liters) and total price paid.
- The tool calculates miles since last fill and MPG automatically.
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 consistent fill-ups in a row averaging around 29.8-29.9 MPG is a reliable real-world baseline for that vehicle under typical mixed driving — useful for comparing against the EPA combined rating printed on the window sticker (often several MPG higher).
What moves your real MPG the most
- City stop-and-go traffic can cut highway-rated MPG by 20-30%.
- Cold weather (below 20°F/-6°C) commonly reduces efficiency 10-15% due to longer warm-up and battery/oil viscosity effects.
- Roof cargo boxes and heavy trailer towing can cut MPG by 10-25% depending on drag and weight.
- Tyre pressure 6-8 psi under spec measurably increases rolling resistance and fuel use.
A single unusually short or long fill-up (partial tank, different pump handle cutoff) will skew that entry's MPG. Look at the trend across several fills rather than reacting to one outlier.
Turning the log into a monthly budget
Once you have 4-6 weeks of entries, the tool totals fuel spend for the period and projects a monthly average — useful for comparing against a prospective EV's charging cost, or for deciding if a shorter commute route or different vehicle would meaningfully cut spend.
Frequently asked questions
How many fill-ups do I need before my MPG number is reliable?
Why is my real MPG lower than the EPA rating on the window sticker?
Does the tool work for diesel and hybrid vehicles?
Can I export my fuel log for tax or business mileage purposes?
Sources & further reading
- U.S. EPA — Fuel Economy testing and window sticker methodology
- U.S. Department of Energy — Fuel economy driving tips
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.
