Within the framework of a Device-to-Device (D2D) data offloading system for cellular networks, we propose a Content Delivery Management System (CDMS) in which the instant for transmitting a content to a requesting node, through a D2D communication, is selected to minimize the energy consumption required for transmission. The proposed system is particularly fit to highly dynamic scenarios, such as vehicular networks, where the network topology changes at a rate which is comparable with the order of magnitude of the delay tolerance. Through extensive system level simulations, we compare the energy consumed by the devices to perform D2D data offloading using the proposed scheme with the energy consumed when using a benchmark scheme (proposed in previous works) without optimal transmission instant selection. The results show that, in specific scenarios, compared to the benchmark system in which the transmission instant is not optimized, the proposed system allows a reduction of the energy consumed for D2D communications above 90%.