Infotainment service is getting popular in vehicular ad hoc networks (VANETs). Through inter-vehicle communication, vehicles can retrieve data from other vehicles, including multimedia data. In this paper, we propose an Overlay Multicast for VANETs (OMV). In OMV, interested vehicles (called group member nodes) join a multicast tree initiated by a vehicle providing multimedia data as the multicast source node, and the group member nodes are organized as an overlay structure. The OMV has a characteristic of dynamically adaptive to the road layout for urban VANETs. We present two new strategies, (1) locally adapting to road intersections and (2) globally probing for a better parent, to support the characteristic. Simulation results show that under a real setting of IEEE 802.11p, a vehicle mobility model, and a urban road layout, for group size under 20, the data delivery ratio can be 0.8 or more.