The main reduction ratio has a direct impact on the structure, outline and mass of the main reducer, and the power performance and fuel economy of the automobile when the transmission is in the highest gear. The choice should be determined by the vehicle power calculation together with the total transmission ratio of the transmission system in the overall design of the vehicle. The power balance diagram under different conditions can be used to calculate the influence on automobile dynamic performance. Through optimal design, the engine and transmission parameters are best matched to select the values, but the automobile can obtain the best power performance and fuel economy