IEEE 802.11 standard provides two access mechanisms for its distributed coordination function (DCF) protocol, the default basic access mechanism and the optional request-to-send/clear-to-send (RTS/CTS) mechanism, respectively. In this paper, we study how to adaptively tune the RTS threshold to guarantee that the network always operates in the access mode that leads to higher network sum rate performance. In contrast to the previous studies based on numerical calculation or channel estimation, an explicit expression of the optimal RTS threshold is obtained in this paper, and verified by simulation results. The analysis shows that with the current fixed standard setting of RTS threshold, IEEE 802.11 DCF networks fail to activate RTS/CTS mechanism properly, leading to sum rate degradation. With the optimal selection of RTS threshold, nevertheless, the network can accurately switching between two access modes, which improves the network sum rate performance for WiFi networks.