Optimal allocation of wireless power transfer system (WPTSys) for electric vehicles (EVs) plays an important role in reducing the manufacturing costs of WPTSys. In the conventional approach, the allocation of WPTSys requires information on EV operation such as speed profile and power demand. This paper proposes a new methodology based on mathematical optimization for the simultaneous design of EV speed profile and allocation of WPTSys in a lane segment, with the aim of reducing the cost of WPTSys by minimizing its length. By parameterizing both EV operation and allocation of WPTSys as unknown parameters, an optimization problem with constraints of EV operation and battery sustainability is established for optimizing these unknown parameters to minimize the length of WPTSys. Therefore, the optimal speed profile of EV and optimal allocation of WPTSys can be simultaneously determined. The proposed approach is applied to a scenario of autonomous driving EVs, where it is assumed that the designed speed profile is accurately tracked. A numerical case study is conducted to illustrate the proposed approach. Furthermore, it is shown that our proposed method achieves WPT systems that are shorter in length compared to those obtained using the conventional method under the same operational constraints of EV and battery.