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.