The core measurement: distance ÷ fuel used
For 500 km covered using 40 liters of fuel: 500 ÷ 40 = 12.5 km per liter. The same trip also converts to 8 liters per 100 km, and 29.4 miles per gallon — three different units expressing the exact same underlying efficiency.
A second, slightly less efficient example
For 600 km covered using 50 liters: 600 ÷ 50 = 12 km per liter — slightly less efficient than the first example, working out to 8.33 liters per 100 km and 28.23 miles per gallon.
Why liters/100km moves in the opposite direction
Km/l and liters/100km measure the same thing but read oppositely: a higher km/l is better (more distance per liter), while a lower liters/100km is better (less fuel needed for the same distance). Going from 12.5 km/l to 12 km/l in the two examples above correctly shows up as liters/100km rising from 8 to 8.33 — consistent, opposite-direction confirmation of the same efficiency difference.
Measuring it accurately
The most reliable method is filling the tank completely, resetting the trip odometer, driving normally, then filling up completely again — the liters needed to refill is the actual fuel used for that distance, rather than an estimate.