The formula
l/100 km = litres ÷ kilometres × 100
Where it comes from
The trip computer tends to flatter: it measures flow at the injector and accounts neither for the fuel left in the circuit nor for the error in the sensor itself. Brim to brim is the only reliable method, because it measures what actually went into the tank.
How to work it out by hand
- Fill the tank until the pump cuts out and zero the trip meter
- Drive normally until you have used at least half a tank
- Fill up again at the same pump and note the litres
- Divide the litres by the kilometres and multiply by a hundred
What is worth knowing
Repeat the measurement at the same pump and with the same idea of full: filling to the first cut-out or to the brim changes the result by a couple of litres, which over 500 km is nearly half a litre per hundred. And bear in mind that real consumption varies a great deal: town driving can double motorway figures, and winter adds 10 % to 20 % from the cold engine.