Vehicular ad hoc networks (VANETs) are characterized by intermittent connectivity, high mobility of vehicle nodes and dynamic topology. This makes data delivery in VANETs very challenging. Pervious works that based on historical traffic pattern or historical data delivery delay to predict current traffic conditions on the roads are not accurate. Deploying roadside units (RSUs) is a possible solution to overcome the challenges, but it often requires investment. Driven by the fact that there are large amounts of outside parked vehicles in urban areas, this paper proposes a parking backbone based data delivery paradigm (PBBD) for VANETs. PBBD does not need any RSUs, but leverages a virtual overlay network formed by outside parked vehicles to help transmitting messages among vehicles. This scheme consists of two parts. First, to each road, parked vehicles both at roadside and off-street are grouped into a cluster as large as possible. An urban overlay network is established based on this type of clusters for data transmission. Secondly, novel message delivery schemes are designed to efficiently transmit messages to destination vehicles through the proposed virtual overlay network. Simulation results based on a real city map and realistic traffic situations show that PBBD achieves a higher delivery ratio with lower network transmission overhead and reasonable transmission delay.