Finding a path with enough throughput in multihop wireless ad hoc networks is a critical task of QoS Routing. Previous studies on routing algorithms focused on networks with a single channel rate. The capability of supporting multiple channel rates, which is common in wireless systems, has not been carefully studied in routing algorithms. In this paper, we first carry out a comprehensive study on the impacts of multiple rates, interference and packet loss rate on the maximum end-to-end throughput or path capacity. A linear programming problem is formulated to determine the path capacity of any given path. This problem is also extended to a joint routing and link scheduling optimization problem to find a path with the largest path capacity. We show that interference clique transmission time is inversely proportional to the upper bound of the path capacity, and hence we propose to use it as a new routing metric. Moreover, we evaluate the capability of various routing metrics such as hop count, expected transmission times, end-to-end transmission delay or medium time, link rate, bandwidth distance product, and interference clique transmission time to discover a high throughput path. The results show that different routing metrics lead to paths with significantly different path capacity, and the interference clique transmission time tends to discover paths with higher throughput than other metrics.