The demand for internet access in vehicles has been growing. To access the internet from vehicles, a wireless road-vehicle communication system is necessary. In conventional road-vehicle communication systems, user terminals in the vehicles have to directly connect to wireless access points (APs). However, the speed of vehicles is so fast that the channel condition between the terminals and the APs constantly changes due to changing path-loss and time-varying fading. In this paper, we used an inter-vehicle packet relay technique to improve channel quality in road-vehicle communication systems. If a terminal can send data via other vehicles moving at lower speed, the relative speed will decrease, which suppresses the dynamic range of path loss and deterioration by fading. We evaluated this method using numerical analysis to verify the availability of the method.