Fill the tank, drive, fill it again and note the kilometres and the litres of that second fill-up. For example, 500 km on 35 litres is 7 l/100 km, or 14.29 km per litre. The tank size and the manufacturer figure are optional.

Educational estimate, not financial advice. Consumption measured over a single tank varies with the route, driving style and traffic; repeat the measurement over several tanks for a reliable figure.