MPG Calculator
What gas mileage is your car really getting? Enter the odometer readings from two consecutive fill-ups (or just miles and gallons) to get your true MPG — plus metric equivalents and what each mile costs you in fuel.
at last fill-up
at this fill-up
gallons to refill the tank
optional, for cost/mile
Fuel economy
29.2
MPG (350 mi ÷ 12 gal)
L/100 km
8.06
12.40 km/L
Gal per 100 mi
3.43
EPA's preferred metric
Fuel cost per mile
12.0¢
at $3.50/gal
For an accurate reading, fill the tank completely at both fill-ups and reset your trip meter — the gallons pumped at the second fill then equal exactly the fuel burned over the miles driven. Gallons per 100 miles scales linearly with fuel use, which is why the EPA prints it on window stickers: going from 15 to 20 MPG saves more fuel than 30 to 40.
Measuring Real-World Fuel Economy
The accurate method is fill-to-full both times: top off the tank, drive normally, then top off again — the gallons pumped the second time are exactly what you burned over the miles in between. Drive 350 miles and refill with 12 gallons, and you got 350 ÷ 12 = 29.2 MPG, which is 8.06 L/100 km or 12.40 km/L. At $3.50 a gallon that's 12.0¢ of fuel per mile. One tank is a sample, not a verdict — average three or four fill-ups to smooth out weather, traffic, and a pump that clicked off early.
Conversions & worked example
MPG = miles ÷ gallons L/100 km = 235.215 ÷ MPG km/L = MPG × 0.4251 gal/100 mi = 100 ÷ MPG cost/mile = price per gallon ÷ MPG 350 mi on 12 gal: 350 ÷ 12 = 29.2 MPG 235.215 ÷ 29.17 = 8.06 L/100 km 29.17 × 0.4251 = 12.40 km/L $3.50 ÷ 29.17 = 12.0¢ per mile
Frequently Asked Questions
Is 30 MPG good?
Yes — the EPA's real-world average for new US vehicles is about 28 MPG, so 30 combined beats the typical new car, and most older vehicles on the road do worse. For context: full-size pickups usually land at 17–20 MPG, midsize sedans 30–36, and hybrids 45–55.
How do I calculate my car's exact MPG?
Fill the tank completely, note the odometer, drive normally, then fill completely again. Miles driven divided by the gallons of the second fill is your true MPG — 350 miles on 12 gallons is 29.2 MPG. The dashboard readout typically flatters by 3–5%, and one tank is just a sample, so average several fill-ups.
Why does the EPA list gallons per 100 miles?
Because fuel use, not MPG, is what scales linearly with cost. Going from 15 to 20 MPG saves 1.67 gallons per 100 miles, while the bigger-sounding jump from 30 to 40 MPG saves only 0.83. This 'MPG illusion' is why window stickers carry the gal/100 mi figure — it makes comparisons honest.
What is 30 MPG in liters per 100 km?
235.215 ÷ 30 = 7.84 L/100 km, or equivalently 12.75 km/L. The formula inverts both units — miles per gallon measures distance per fuel, while L/100 km measures fuel per distance — which is why higher MPG means a lower L/100 km figure.