Currently a number of 802.11 based standards support multiple rates at physical layer and recently 802.11 MAC protocols have been modified to make use of these multiple rates. Two such mechanisms, opportunistic auto rate (OAR) and receiver based auto rate (RBAR) have shown to improve throughput performance of wireless LANs as well as infrastructure-less wireless networks. However these protocols are unable to adapt to short-term changes in channel conditions during transmission as well as unable to use optimum power and throughput during packet transmissions. In this paper, an improved opportunistic auto rate protocol (IOAR) has been proposed as an efficient rate adaptive MAC. Two subtypes of IOAR based on power control requirement have been proposed. Using an event based simulation we show that the proposed schemes outperform the existing rate adaptive MAC protocols such as RBAR and OAR, in terms of throughput by upto about 24%.