Incompletely predictable vehicular ad hoc networks (IPNs) are presented for vehicular networks with vehicles moving in particular ranges. A group of vehicles in the same region are required to encrypt and sign the uploaded region information in IPNs. So far, research has not been able to provide an appropriate trustworthiness evaluation system ensuring security and fairness in IPNs. In this paper, we first establish an evaluation system for vehicles in the network for secure and fair trustworthiness evaluation. Various vehicle attributes are utilized to evaluate the trustworthiness level of a vehicle. The novel trustworthiness evaluation can provide a more comprehensive and real-time update of vehicle status. Second, based on the system, we propose a cloud-aided trustworthiness evaluation (CATE) scheme. According to the evaluation results provided by the system, the CATE scheme also guarantees lightweight trustworthiness level confirmation with the help of session key generation. Besides, the information upload security is ensured with group signature and encryption. The security proof indicates that the proposed CATE scheme is secure for vehicles in IPNs to interact with trustworthiness. According to quantitative comparative analysis and experimental simulation based on paring-based cryptography (PBC) library, the designed scheme has been proved to be efficient.