Real consumption in l/100 km from the distance driven and the litres filled.
Fill the tank, zero the trip meter, drive your usual route and fill it again. Divide the litres put in by the kilometres driven and multiply by a hundred. Both fills must be at the same station and up to the automatic cut-off, or the measurement is not comparable.
Seven litres per hundred kilometres is about 40 miles per imperial gallon, or 34 per US gallon. The conversion is not linear, because l/100 km and mpg are reciprocal: fewer litres means more miles.
The trip computer does not measure consumption; it calculates it from injector opening time, and manufacturers often calibrate that optimistically. The gap is usually 3–7%. The tank-to-tank method is more accurate because it counts litres actually paid for.
Speed above a hundred kilometres an hour, because drag rises with the square, a roof rack that adds up to 20%, under-inflated tyres, and short trips on a cold engine. The first two kilometres in winter can burn twice what a warm engine does.