MPG Calculator
The dashboard lies a little. Hand-calculate your true MPG from the odometer and the pump, then compare it against what the car claims.
Hand-calculated MPG28.5 mpg
Metric equivalent8.3 L/100km
MPG = (odometer now − odometer at last fill) ÷ gallons to fill · L/100km = 235.21 ÷ mpg
The method that matters: fill to the same automatic stop each time, note the odometer, and repeat over 3+ tanks. Single-tank numbers bounce around with pumps and weather; the multi-tank average is your real number. Apps like Fuelly exist for exactly this.
Dash optimism is normal: if your dashboard reads more than ~3% above the pump math, that is the common pattern — not a fault. Speedometer error from non-stock tires feeds the estimate too. See the hand-calculated MPG guide.
Assumptions & sources
MPG formulaΔmiles ÷ gallonsFill-up method, same-pump-stop convention
US→metric constant235.2145831 US gal = 3.785411784 L; exact
Bias flag threshold> 3% dash-over-pumpr/f150 recurring owner consensus
Frequently asked questions
How do I calculate MPG by hand?
Reset or note the odometer at a fill-up. At the next fill-up, subtract: miles driven ÷ gallons pumped. 350 miles on 12.3 gallons = 28.5 mpg. Repeat over several tanks for a stable number.
Why is my calculated MPG lower than the dashboard?
Dashboard estimates commonly skew optimistic — owner communities measure 3–8% gaps routinely. Speedometer error from bigger tires and pump shut-off variation both feed the gap. The pump math is the reference.
How do I convert MPG to L/100km?
Divide 235.21 by your MPG: 28.5 mpg = 8.3 L/100km. Going the other way, 235.21 ÷ 8.3 = 28.3 mpg. The two units move in opposite directions — lower L/100km is better, higher MPG is better.
How accurate is one tank of gas for MPG?
Single-tank readings swing with pump stop points, weather, and traffic — treat ±1 mpg as noise. Three to five consecutive tanks averaged together give you a number you can budget on.