In order to extend the usable range of automobile transmission parameters by satisfying the performance requirements of the vehicle, a new optimum model is presented. In this model, design variables become interval. The aim of optimization is to find maximum feasible interval of design variables. It can successfully solve the question about the mating gears in optimum design of transmission parameters. A typical interval optimum example of the transmission parameters of an urban bus is given in this paper. The optimum results indicate that the fuel economy of the bus is improved obviously,and the dynamic performance is not notably changed in contrast with results of the original vehicle. The method proposed in this paper is not only adapted to optimal design of automobile transmission parameters, but also in other engineering design fields.