The increased public interest in economic use of energy has lead to intense efforts by engineers to achieve a reduction in engine friction losses. Despite differences quoted in the literature, the proportion of the friction losses caused by the piston assembly is certainly of such an order as to justify considerable expenditure in the numerical determination of consumption improvements due to optimization of piston and rings. In engine tests carried out by Kolbenschmidt AG an accuracy of ≤ ± 0,5% has been achieved in the measurement of specific fuel consumption. This paper presents a basic description of the measurement method and the results of piston optimization for several small engines (four cylinder gasoline and diesel engines). A 3% - 5% fuel economy improvement under combined city/highway traffic conditions can be achieved by the use of an optimized Fuel Economy Piston. The contribution of the basic design parameters such as weight, skirt area and ring pack are discussed.