The subject of this paper is high-speed induction motor (IM) with drag-cup rotor with low inertia, designed for use in high-speed automotive applications such as kinetic energy recovery system (KERS). The aim of this paper is to present the optimization procedure of the air-gap length and the rotor thickness of this kind of motor in order to achieve the highest efficiency in the speed and torque range of interest. Simple procedure for stator dimensioning was conducted in order to get main dimensions of the motor. Based upon reasonable approximations, analytical considerations are performed in order to envisage and demonstrate the effects of changes in the rotor thickness and the two air-gap lengths on the motor performance. Starting with these raw results, finite element method (FEM) tools are used to obtain more precise performance prediction, and to perform the optimization of the air-gap lengths and the rotor thickness. Finally, simulation results were consolidated, presented, and used to draw conclusions and propose design guidelines.