Calculate fuel economy in km per liter.
Fuel economy in metric markets is quoted two ways that are reciprocals of each other. Kilometres per litre behaves like mpg — higher is better — while litres per 100 km is consumption, where lower is better, and it is the form most European manufacturers and regulators use because it is linear in fuel: halving L/100km halves the fuel bill, whereas doubling km/L does not double the improvement in a comparable way. Measure it by brim-to-brim filling: fill the tank, reset the odometer, drive, then refill to the same point and record the litres. One tank is noisy; three consecutive tanks averaged is a figure you can trust.
Kilometres per Litre
km/L = distance / litres; L/100km = 100 / (km/L); US mpg = km/L x 2.35215; cost per km = litres x price / distance
L/100km for comparing and budgeting, because it is proportional to fuel used. km/L is more intuitive as an efficiency score but distorts comparisons between very different vehicles.
Trip computers estimate from injector duty and are commonly optimistic by 2-8%. The brim-to-brim method measures what you actually bought, so trust it over the display.