Nowadays, with the purpose of fulfilling the increasing demands for emerging extensive and ubiquitous accessibility to modern intelligent transportation systems (ITS), the conventional vehicle-to-everything (V2X) paradigms are progressively evolving into the Internet of vehicles (IoVs). As of now, substantial enhancements in IoV technologies have been witnessed, particularly in the era of vehicular data secure exchange and user privacy protection. On the other hand, unmanned aerial vehicles (UAVs) are envisioned to provide scalable and adaptive stereoscopic service coverage for IoV implementations. However, the existing approaches intended for the common scenarios mainly rely on the well-established local IoV infrastructures, whereas the specific infrastructure-less IoVs with dysfunctional edge facilities have not been properly investigated. Additionally, the related UAV-assisted IoV schemes still fail to independently perform the secure vehicular data transmission but rather as the auxiliary strategies due to the dependence on the assistance of local edge deployment. Therefore, the practical requirements of reliability and scalability in real-world IoV circumstances cannot be satisfied. In this paper, an efficient UAV certificateless group authentication mechanism is developed in order to facilitate the secure data transmission of infrastructure-less IoV. The proposed design deploys the tethered UAV (TUAV) as the specific mobilized base station so that the active edge IoV infrastructure is not needed. The security analysis regarding the key security features are presented first, followed by the performance evaluation. According to the comparison results with the existing designs, improvements in terms of computational cost and communication overhead for different phases can be demonstrated.