Fuel consumption is one of the most important operating characteristics of road vehicles. In operation of road vehicles there we usually express fuel consumption in l/100 km for driving performance, or l/h when the car engine is idling. The fuel consumption can be quantified by several methods, and it affects a number of factors (driver, vehicle, and environment). It can be considered as a posteriori consumption, i.e. operational consumption. The priori fuel consumption (inserted, pre-defined) based on the construction properties of respective engine. Most often it is the priori fuel consumption expressed by measuring the effective consumption, which defines the amount of fuel consumed per unit of effort and time. It is usually quantified at a time when the engine is mounted to the vehicle itself. The contribution presents a practical example of measuring the effective fuel consumption in operation of road vehicles.