This paper has reviewed the performances of various AGCs which can be adopted in IEEE802.11p modems. IEEE802.11p, a high speed mobile communication standard for vehicles, requires high performance signal detector since the channel impulse responses are varied rapidly in time. In order to select the optimal signal detector, we simulated the performances of three detection methods. One is using RSSI signal, the other is using RSSI signal and I/Q signal, and the third is using I/Q signal through the Monte Carlo simulation. We evaluated the performances of the algorithms using our own system based on MAX 2829 transceiver(MAXIM $Integrated^{TM}$) in a real vehicular environment. As a result, the experiment using Fully I/Q signal derives the most excellent performance with the lowest minimum receiver sensitivity, packet error rate (PER) and false alarm rate (FAR).