How fuel economy calculator works
This calculator figures out your real-world fuel economy from two numbers you read off a fill-up: the distance you traveled since the last fill and the amount of fuel it took to refill the tank. Enter both and you get every common efficiency unit at once.
Behind the scenes it normalizes units so the four economy figures agree. Distance converts between miles and kilometers (1 mile = 1.609344 km) and fuel volume between gallons and liters (1 gallon = 3.78541 liters). MPG is miles divided by gallons; km/L is kilometers divided by liters; L/100km is 100 divided by km/L. The math is the inverse of the fuel cost calculator, which starts from a known efficiency.
Optionally add a fuel price to project the trip cost and your cost per mile or kilometer at this economy. Because real economy varies with speed, load, weather and terrain, treat the result as a fill-up-to-fill-up average rather than a fixed spec.
Frequently asked questions
How do I calculate MPG after a fill-up?
Fill your tank completely, reset the trip odometer, drive normally, then fill up again. MPG is the miles on the trip odometer divided by the gallons you just added. This calculator does that division and also shows L/100km and km/L.
How are MPG and L/100km related?
They are inverses. MPG is distance per unit of fuel (higher is better); L/100km is fuel per unit of distance (lower is better). To convert: L/100km = 235.215 divided by MPG (US gallons). This tool computes both from the same fill-up so they always agree.
Can I enter kilometers and liters instead?
Yes. Choose kilometers for distance and liters for fuel. The calculator still shows all four economy figures, so you can read MPG, km/L, mi/L and L/100km from one metric fill-up.
Why is my MPG lower than the advertised number?
Manufacturer figures are lab-tested under ideal conditions. Real economy drops with city driving, heavy loads, cold weather, roof cargo, under-inflated tires and aggressive acceleration. Track several fill-ups and average them for a realistic number.
