Method for determining the performance of a motor vehicle consumable, including engine lubricant, fuel and tires in a motor vehicle having an engine with at least one fuel injection valve, a lubricating system and a CANbus network having at least three network nodes, each operably connected to a computer. The method includes operating the vehicle while supplying the engine with fuel through a fuel injection valve and lubricating the engine with the engine lubricant. A first CANbus network node transmits to the computer a signal indicative of the engine fuelling rate based upon the opening time of a fuel injection valve. One or more second CANbus network nodes transmit signals to the computer indicative of one or more variables indicative of the power output of the engine, and one or more third CANbus network nodes transmit signals to the computer indicative of the vehicle operating conditions. The computer calculates a corrected power output indicative of the power output of the engine corrected for the vehicle operating conditions using the one or more signals transmitted to the computer by the second and third CANbus network nodes and a pre-determined algorithm, and the computer transmits and/or stores the corrected power output and the signal indicative of the engine fuelling rate.