Gas Mileage Calculator
Measure your real-world gas mileage. Enter two odometer readings (or the trip distance) and the fuel it took to refill the tank to get MPG, the metric equivalents, and your cost per mile — benchmarked against the EPA average for new vehicles.
Fill the tank completely both times — the gallons added on the second fill equal the fuel burned over the miles in between.
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Compare ratesHow to measure gas mileage properly
The tank method is the gold standard: fill up completely, note the odometer (or zero the trip meter), drive normally, then fill up completely again. The gallons the pump adds on the second fill are exactly what you burned over those miles, so MPG = miles ÷ gallons. One tank has some noise — weather, terrain, a heavy week of city driving — so average three or four fills before judging the car.
Dashboard MPG displays commonly read 3–5% optimistic, which is why the odometer method is used by fleet managers and hypermilers alike. Cost per mile is the number that makes commutes and road trips comparable across cars.
How it’s calculated
MPG = distance ÷ fuel. Metric: L/100 km = liters × 100 ÷ km (lower is better) and km/L = km ÷ liters. Cross-conversion: L/100 km = 235.215 ÷ MPG(US). Cost per mile = fill cost ÷ miles driven.
MPG varies with speed, load, tires, and season — a single tank is an estimate; average several fills for a fair picture.
Worked example
Odometer 38,450 → 38,762 is 312 miles. The pump takes 11.4 gallons at $3.10: that’s 27.4 MPG (8.6 L/100 km), a $35.34 fill, and about $0.11 per mile in fuel — almost exactly the 27.2 MPG that new U.S. vehicles average in EPA real-world data. In metric mode, 520 km on 38 L works out to 7.3 L/100 km (32.2 MPG).
Common mistakes
- Not filling the tank fully both times — partial fills make the gallons figure meaningless.
- Trusting the dash readout; trip-computer MPG typically runs a few percent high.
- Judging the car on one winter tank — cold starts and winter fuel blends cut MPG roughly 10–20% in town.
- Comparing MPG to a UK figure — imperial gallons are 20% larger, so 40 MPG (UK) ≈ 33 MPG (US).
Where it is used
- Tracking whether a tune-up, new tires, or driving style changed your economy.
- Working out true commute and road-trip fuel cost per mile.
- Validating a used car’s window-sticker claims before buying.
- Fleet and rideshare per-mile expense tracking.
Frequently asked questions
Is my MPG good?
New U.S. light-duty vehicles averaged a record 27.2 MPG in real-world EPA data for model year 2024 (Automotive Trends Report). Pickups and older SUVs commonly sit in the teens; hybrids run 40–55. The compare bar above scores your number against the fleet average.
Why is my MPG worse than the window sticker?
EPA label values come from standardized lab cycles. Short trips, cold weather, roof racks, low tires, and speeds over 65 mph all cut real-world economy — 10–20% below sticker is common in mixed driving.
How do MPG and L/100 km relate?
They are reciprocals: L/100 km = 235.215 ÷ MPG(US). So 25 MPG is 9.4 L/100 km and 8 L/100 km is 29.4 MPG. Note the direction flips — lower L/100 km is better.
Does the price field change the MPG?
No — price only feeds the cost lines (cost per mile and fill-up cost). Leave it blank if you just want fuel economy.
How can I improve my gas mileage?
The big levers: keep tires at spec (each PSI low costs about 0.3%), slow down (fuel economy falls quickly above about 55–60 mph), remove roof racks and dead weight, avoid hard acceleration, and fix any check-engine faults like a bad oxygen sensor.