Gas Mileage Log Calculator

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)

Note carefully what that is not. It is not the average of your per-tank MPG figures. Those two disagree whenever the fills are unequal, because a bigger tank should count for more. Three tanks at 28.0, 29.2 and 26.0 average out to 27.73 either way here only because the fills happen to be similar; make one of them a splash-and-dash of two gallons and the naive average will flatter or punish you by a full MPG. Total distance over total fuel is always the honest figure.

How to log so the numbers mean something

Fill to the same point every time, ideally the first click of the pump rather than topping off, since how far you top off is otherwise the largest source of noise in the whole exercise. Reset the trip meter at each fill, or note the odometer and subtract. Record the fill even when you did not fill completely, because the fuel still went in and the distance still got driven, and over three or four tanks the errors wash out.

One tank tells you almost nothing. It tells you what that week’s driving looked like, and a week of motorway commuting against a week of school runs can differ by a third. Three or four tanks is where a number starts being about the car rather than the weather.

Fuel Cost Per Mile:

Cost/Mile = Fuel Price / MPG

Monthly Fuel Spend:

Monthly Cost = Monthly Miles / MPG × Fuel Price

Worked example, three consecutive fill-ups:

• 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.