Among the proposed opportunistic content sharing services, Floating Content (FC) is of special interest for the vehicular environment, not only for cellular traffic offloading, but also as a natural communication paradigm for location-based context-aware vehicular applications. Existing results on the performance of vehicular FC have focused on content persistence, without addressing the key issues of the effectiveness with which content is replicated and made available, and of what are the conditions which enable acceptable FC performance in the vehicular environment. This work presents a first analytical model of FC performance in vehicular networks in urban settings. It is based on a variation of the random waypoint (RWP) mobility model, and it does not require a model of road grid geometry for its parametrization. We validate our model extensively, through numerical simulations on real-world traces, showing its accuracy on a variety of mobility patterns and traffic conditions. Through analysis and simulations, we show the feasibility of the FC paradigm in realistic urban settings over a wide range of traffic conditions.