The majority of methods which optimize the power plant to automobiles utilize on one hand vehicle test-run simulation programs, and on the other hand optimization programs. This paper proposes more synthetic techniques which use operationnal characteristics proper to the engine and to the vehicle.This adaptation always promotes the utilization of the longest ratio and improves the mechanical efficiency of the engine. These techniques show clearly the different reasons of losses, and where they happen. It prevents also the utilization of unoptimized components to built a complete vehicle.Maximal speed and accelerations determine the displacement of the engine and the ratio of the fourth gear. The starting uphill performances determine the longest first ratio. The second and third gears are inserted within. The consumption or the performances of this vehicle can be then evaluated in advance.