This study proposes a practical process for optimizing powertrain systems. The method used mixed-integer linear programming to optimize the control and analyze the sensitivity of the design parameters in a given power train structure. Modifying the variable with the highest sensitivity led to the optimal design solution because the magnitude of the sensitivity indicated the importance of the variable for satisfying vehicle performance criteria. The proposed process was applied to the gear-ratio design of an electric vehicle system, and the optimal gear ratios were determined. The proposed process contributes to the realization of carbon-neutral society because it facillitates vehicle electrification and can reduce the number of iterations for developing prototypes.