Fuel Economy MPG Calculator
Calculate your real-world miles per gallon (MPG), litres per 100 km (L/100 km), and fuel cost per mile. Fill your tank completely, drive normally, then fill again and note how many gallons were added and how far you drove. Enter those numbers below. The EPA uses this same tank-to-tank method when verifying fuel economy data for fueleconomy.gov. You can also enter a fuel price to see your cost per mile and per kilometre.
Fuel economy formulas
MPG = Miles_driven / Gallons_used
L/100 km = 235.214 / MPG
Cost per mile = (Fuel_price / MPG)
Cost per km = Cost_per_mile / 1.60934
The conversion constant 235.214 is derived from the US gallon (3.78541 L) and the miles-to-km factor (1.60934). This matches the formula published on EPA's fueleconomy.gov.
Tips to improve fuel economy
- Keep tires inflated to the pressure on the door-jamb placard. The EPA states that every 1 PSI below recommended pressure reduces MPG by about 0.5%.
- Avoid aggressive acceleration and hard braking. Smooth driving can improve fuel economy by 15 to 30% on the highway per fueleconomy.gov.
- Remove unused roof racks and cargo carriers. A rooftop box can reduce highway MPG by 25%.
- Avoid extended idling. Idling gets 0 MPG; restarting a warm engine uses less fuel than 10 seconds of idling.
- Use the recommended motor oil grade listed in your owner's manual. Using 10W-30 instead of 5W-30 can lower MPG by 1 to 2%.
Fuel economy MPG calculator: frequently asked questions
How is miles per gallon calculated?
MPG = miles driven / gallons of fuel used. Fill the tank to the brim, drive a measured distance, then fill again and note the gallons added. Divide miles by gallons. This is the standard tank-to-tank method used by the EPA for verifying manufacturer fuel-economy claims.
How do I convert MPG to litres per 100 km?
L/100 km = 235.214 / MPG. This conversion factor comes from the relationship between US gallons (3.78541 litres) and miles-to-kilometres (1 mile = 1.60934 km). For example, 30 MPG equals 235.214 / 30 = 7.84 L/100 km.
What is a good MPG for a car?
The EPA reports the average fuel economy of new model-year vehicles each year. For 2023 model year vehicles the fleet average was about 26.4 MPG (combined city and highway) per EPA's Automotive Trends Report. A result above 35 MPG combined is considered good; above 45 MPG is excellent.
How does city vs highway driving affect MPG?
City driving involves frequent stopping, idling, and low-speed acceleration, all of which reduce fuel efficiency. The EPA typically tests city MPG at an average speed of about 21 mph and highway MPG at an average of about 48 mph. Most vehicles achieve 20 to 30% better fuel economy on the highway than in the city.
What factors reduce real-world MPG below the EPA estimate?
Cold weather, short trips, aggressive driving, roof racks, underinflated tires, and carrying extra weight all reduce MPG. The EPA's fueleconomy.gov notes that aerodynamic drag from a roof-top cargo box can cut highway fuel economy by 25%. Underinflated tires can lower MPG by about 0.5% for every 1 PSI drop.
Official sources
- EPA Fuel Economy: fueleconomy.gov (fuel economy data and tips).
- EPA Automotive Trends Report: epa.gov/automotive-trends.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.