How gas mileage calculator Works
The fill-up-to-fill-up method
The only measurement that doesn't lie: fill the tank, reset the trip odometer, drive normally, and at the next fill-up divide trip miles by gallons pumped. The pump meter and the odometer are calibrated instruments; your dashboard's economy readout is an estimate that's usually 5–10% optimistic.
How to measure it properly
- Fill the same way every time — same click-stop on the nozzle. Variations in fill level become variations in "gallons used."
- Ignore the first tank after resetting anything; use tanks 2–4 as your average.
- Average across several tanks. Single-tank MPG swings with highway vs city mix, weather, and cargo.
Why cost per 100 miles matters
MPG is unintuitive for budgeting: the difference between 20 and 25 MPG sounds similar to 30 vs 35, but the first saves $0.0125/mile while the second saves $0.0048 — less than half. Cost per 100 miles makes comparisons linear: at $3.45/gallon, a 20 MPG car costs $17.25 per 100 miles; a 30 MPG car, $11.50. Multiply by your monthly miles and the calculator's monthly estimate is exactly what your budget sees.