The article deals with the evaluation of mechanical efficiency and with calculation and measurement of overall mechanical losses at spark-ignition, four-stroke petrol engines with power output 40 - 50 kW for small passenger cars. The principal distribution of overall mechanical losses (divided into the main partial losses, like frictional losses, cylinder charge exchange losses or accessories input power losses) is focused. In the case of the engines for small cars there are two actual topics. The first one is the mechanical loss on the valve gear train, especially by using hydraulic tappets and the second one is being all losses caused by engine-driven accessories, like compressor for air conditioning, hydraulic pump for power steering, electrical AC generator, ABS brake system, luboil pump, water pump, etc. The results show that there is not a big and significant difference in mechanical losses due to the principal design of the valve gear train, like OHV and OHC but using roller tappet the losses of the valve gear train are about one-half lower. The second problem of power input of accessories is very important especially at idling a cool engine. There is a strong need to find other ways for the drive of accessories, like power-by-wire.