Opportunistic routing schemes usually infer future contact time to select next hop relays. But the effect of relay selected in this way is uncertain. In order to get further understanding of the intrinsic uncertainty characteristic of relay efficiency, this paper makes an empirical investigation on opportunistic forwarding relay selection schemes using contact records. Based on the underlying opportunistic vehicular network extracted from large-scale realistic vehicle traces, we evaluated the efficiency of relay selection and got some intrinsic sight of opportunistic forwarding. The questions we investigated are: What is the probability that a selected relay can make the end-to-end delay reduced? How much the latency can be saved by a properly selected relay? Such an empirical study is meaningful for the protocol design and applications deployment in future opportunistic networks.