Fuel Economy Calculator
Turn distance and fuel used into MPG and L/100 km at once, so you can read your car’s efficiency either way.
How it works
MPG = miles ÷ gallons · L/100 km = litres ÷ km × 100 (1 mi = 1.609 km, 1 gal = 3.785 L)
Miles per gallon is distance divided by fuel — higher is better. The metric measure inverts the idea: litres per 100 km is fuel divided by distance, so lower is better. Because they run in opposite directions, a car with double the MPG uses half the L/100 km, and small MPG gains at the low end save far more fuel than the same gain at the high end.
Worked example
Driving 350 miles on 12.5 gallons gives 350 ÷ 12.5 = 28.0 mpg. Converted, that is 563 km on 47.3 litres, or (47.3 ÷ 563) × 100 = 8.40 L/100 km.
Frequently asked questions
Is a higher or lower number better?
For MPG, higher is better — more miles per gallon. For L/100 km, lower is better — less fuel per distance. They measure the same efficiency from opposite directions.
How do I measure my real economy?
Fill the tank, reset the trip meter, drive normally, then refill and note the gallons added and miles driven. Dividing miles by gallons gives your true economy over that tank, which beats the dashboard estimate.
Why isn’t doubling MPG a linear fuel saving?
Because fuel used is inversely proportional to MPG. Going from 15 to 20 mpg saves more fuel over a fixed distance than going from 40 to 45 mpg, even though both are a 5-mpg gain — which is why L/100 km is the more intuitive measure of consumption.
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This calculator is for educational purposes. Double-check important results before acting on them.