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.