The relative velocity of the vehicle ad hoc network that has high mobility can affect network topology. This can affect the efficiency of those networks, so a suitable mechanism is necessary for effective management of them. This paper analyzes MAC throughputs in according with relative velocity in vehicle ad hoc environment. The vehicle ad hoc network has serious a topological variation because of high-mobility of a vehicle. So each station has a MAC-overhead to avoid collision. The RTS-CTS frame is the biggest overhead between them. In this paper, we evaluate basic mechanism and RTS-CTS mechanism about sent frame length. The result shows that RTS-CTS mechanism exhibits superior efficiency in low relative velocity and basic mechanism appears better efficiency than that in high relative velocity. Therefore, we confirm that relative velocity is an import factor in vehicle ad hoc network, and propose requirement of suitable mechanism.