The evolution of smart city services and applications requires a more efficient wireless infrastructure to provide the needed data rate to users in a high-density environment with high mobility, satisfying at the same time the request for high-connectivity and low-energy consumption. To address the challenges in this new network scenario, we propose to opportunistically rely on the increasing number of connected vehicles in densely populated urban areas. The idea is to support the macro base station (BS) with a secondary communication tier composed of a set of smart and connected vehicles that are in movement in the urban area. As a first step towards a comprehensive cost-benefit analysis of this architecture, this paper considers the case where these vehicles are equipped with femto-mobile access points (fmAPs) and constitute a mobile out-of-band relay infrastructure. We first study this network system with a continuous time model, in which three techniques to select an fmAP (if more than one is available) are proposed and the maximal feasible gain in the data rate is characterized as a function of the vehicle density, average vehicle speeds, handoff overhead cost, as well as physical layer parameters. We then introduce a time slotted model, in which we consider a more realistic communication channel, with an exponential path loss model, and we investigate the tradeoff between energy consumption and expected data rate, as a function of the system parameters. The analytical and simulation results, with both the continuous and time slotted models, provide a first benchmark characterizing this architecture and the definition of guidelines for its future realistic study and implementation. (C) 2018 Elsevier B.V. All rights reserved.