This paper investigates the Optimal Cooperation Scope (OCS) for Cooperative Bandwidth Sharing (CBS) in vehicular networks. Three relay schemes based on optimal single path relay and opportunistic relay for multihop CBS are proposed and studied. The OCS is solved as the maximum physical distance or hops between the bandwidth requester and bandwidth helper in the optimal solution to maximize the CBS benefit. OCSs for different environments and with different relay schemes are given in the paper. The results indicate that the OCS is sensitive to the cost weight and may also be larger when using a proper opportunistic relay based scheme than using the optimal single path based one.