How to use the mpg calculator
Divide miles driven by gallons used. For a useful real-world estimate, measure across a full tank or several fill-ups rather than a very short drive.
MPG formula
The calculator also shows gallons per 100 miles, which can make fuel consumption easier to compare because lower values are better.
Example: Isaac measures fuel economy
Isaac fills the tank, drives 360 miles, and needs 12 US gallons to refill to the same level. His measured fuel economy is 360 ÷ 12 = 30 MPG.
Using consistent fill levels improves the comparison. Several full-tank measurements can give a more representative average than one short trip.
How to calculate MPG from a fill-up
A common real-world method is to fill the tank, reset the trip odometer, drive normally, then refill the tank. Divide the miles driven by the gallons required to refill the tank.
That result estimates fuel economy over the measured interval. It is more reliable when both fill-ups are comparable and the trip distance is large enough that small pump differences do not dominate the calculation.
One tank is a snapshot, not a permanent MPG rating
Weather, traffic, route mix, tire pressure, fuel blend, idling, cargo, and driving speed can make one tank unusually good or bad. Track several fill-ups if you want a more stable picture of long-run fuel economy.
The calculator reports the arithmetic from the miles and gallons entered; it does not adjust for driving conditions.
U.S. MPG is different from liters per 100 kilometers
Miles per gallon increases as fuel efficiency improves. Liters per 100 kilometers moves in the opposite direction because lower fuel consumption is better. The units are not converted by a simple constant multiplier because one is distance per fuel and the other is fuel per distance.
Assumptions and limitations
What the estimate assumes
Pump shutoff differences, idling, driving conditions, and incomplete fill-ups can affect tank-to-tank measurements.
MPG Calculator FAQs
How do I measure MPG accurately?
Fill the tank, reset the trip odometer, drive normally, then refill and divide the miles driven by the gallons added.
Is higher MPG better?
Yes. Higher MPG means more miles traveled per gallon of fuel.
What is gallons per 100 miles?
It is the number of gallons needed to travel 100 miles. Lower is better.
Does weather affect MPG?
Yes. Temperature, traffic, speed, wind, terrain, tire pressure, and vehicle load can all affect real-world fuel economy.
How is MPG calculated?
Divide miles driven by gallons of fuel used. If you drove 360 miles and used 12 gallons, the result is 30 MPG.
Why does my calculated MPG differ from the car display?
Vehicle displays use onboard estimates, while a fill-up calculation depends on pump volume and trip distance. Differences in measurement and driving conditions can produce different results.
Measured fuel economy can vary from official ratings and from one tank to the next.