Gas Mileage Log Calculator
Track fuel efficiency per fill-up from odometer readings and gallons or liters.
Returns MPG, L/100km, fuel cost per mile, and rolling average efficiency.
Fuel economy tracking helps identify mechanical issues, driving habit changes, or seasonal patterns. The core formula is:
MPG = Miles Driven / Gallons Used
For each fill-up entry:
Miles Driven = Odometer End − Odometer Start
Gallons Used = Fuel Added at Fill-up (assumes filled to full each time)
Running Average MPG:
Average MPG = Total Miles / Total Gallons (tracked over multiple fill-ups)
Fuel Cost Per Mile:
Cost/Mile = Fuel Price / MPG
Monthly Fuel Spend:
Monthly Cost = Monthly Miles / MPG × Fuel Price
Worked Example — 3 consecutive fill-ups:
| Fill-up | Odometer | Miles | Gallons | MPG |
|---|---|---|---|---|
| 1 | 45,000 → 45,387 | 387 | 13.8 | 28.0 |
| 2 | 45,387 → 45,802 | 415 | 14.2 | 29.2 |
| 3 | 45,802 → 46,195 | 393 | 15.1 | 26.0 |
- Total: 1,195 miles / 43.1 gallons = Average 27.73 MPG
- At $3.45/gal: $0.1245/mile
- Monthly cost (1,200 miles/month): 1,200 / 27.73 × $3.45 = $149.35/month
Warning Signs: A drop of >2 MPG sustained over 3+ fill-ups often indicates: underinflated tires (−0.5 MPG per 10 psi low), dirty air filter (−1–2 MPG), failing O2 sensor (−3 MPG), or spark plug issues (−2–4 MPG). Tracking your log regularly catches these problems early and saves money on both fuel and repairs.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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