Ad hoc networks provide data communication services without being bound to an existing infrastructure. An exciting application of highly dynamic ad hoc networks is the exchange of data between running vehicles. For air-interfaces based on a slotted Time Division Multiple Access (TDMA) scheme, slot synchronization is required among the rapidly moving vehicles. In this paper a scheme for completely decentralized slot synchronization of nodes in an Inter-Vehicle Communication (IVC) environment is presented. To evaluate the performance of the proposed mechanism, the influence of real synchronization to the network throughput in case of a slotted ALOHA medium access protocol is investigated. Simulation results indicate only minor throughput degradation compared to an ideal slot synchronization.