Mechanical Efficiency is nothing but the measured efficiency delivered by the engine to the actual efficiency of the engine. Due to the various factors such as wear and tear, exhaust, coolant, operation of valve, bearing, etc. the engine is not able to deliver the 100 percent power that was produced. The designer of the vehicle understand that ideal efficiency cannot be achieved in real world, but still based on the maximum result they get from the performance they proffer the mechanical efficiency to the people. The 100 percent mechanical efficiency can only be achieved when the output power of the vehicle will be same as the input energy of the engine. While moving the energy from engine to the wheels through the transmission and bearing, some of the power gets lost causing the output of less mechanical efficiency.