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Geo cast routing in vehicular ad hoc networks plays an important role as the basis of applications such as traffic information sharing, emergency alarming, and geographic advertisement. It is quite challenging, however, to geo cast packets through multi-hop relay vehicles because of the highly dynamic network topology, large scale city road system and fast moving vehicles. Our idea is to measure vehicles' coverage capability and forward packets to those vehicles with higher probability to successfully deliver the packets. The idea is rooted in the widely accepted concept that vehicular trajectories improve packet routing and the fact that vehicular trajectories are nowadays available through widely used navigation system. To accomplish the idea, the difficulty is to measure the coverage capability of a vehicle over a specific region with only partially available vehicular trajectories without accurate timing information. We propose a novel coverage graph to maintain collected trajectories of all the encountered vehicles and their most update timing information so that the extended coverage capability of each vehicle can be estimated. The coverage graph is constructed in a distributed way based on locally shared information and the packet forwarding decisions can be adaptively made to meet different routing objectives.