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Motorcycles have been classified by engine displacement from commercial and legal demands. If this restriction on displacement were removed, what could be done to improve fuel economy? “Down sizing” would be an obvious idea. However this paper proposes a quite inverse concept, i.e. a large displacement lower speed engine. Mechanical friction in four-stroke engines generally decreases in proportion to the square of engine speed. Thus mechanical efficiency should be better at lower operation speeds. According to this concept, developing a 200cm3 experimental engine was successful in improving fuel economy by 79% in the ECE40 mode and by 97% at 60km/h cruise condition, compared to the 100cm3 original engine.