Significant fuel savings can be achieved by deliberate design measures on the drive line of commercial vehicles. This paper describes some of the results gained by M*A*N in the course of drive line optimization.First the engine and its accessories will be dealt with. Taking a turbocharged and a naturally aspirated engine as examples, three development steps are explained for each of these versions, and their effects an fuel consumption discussed. Then follows a consideration of the savings potential still possible through optimized design of engine accessories.Transmission design and the selection of suitable rear axle ratios for heavy commercial vehicles is also considered.Finally the results of tests to determine the effect on fuel consumption of drive line optimization are discussed. Tests with heavy long-haul road trains with various drive line configurations are considered, then the possibilitie of influencing fuel consumption on city buses by drive line optimization are investigated.