Embedding sensors in urban buses is a promising strategy to deploy city-wide wireless sensor networks. By taking advantage of the mobility of buses, it is possible to achieve extended spatial coverage with fewer sensors as compared to a static setup. The trade-offs are that urban buses only cover part of the city, and that the frequency of the buses, and consequently of the data collection, is inhomogeneous across the city. Depending on the communication technology, buses may be unable to deliver the collected data on time. In this paper, we propose a coverage metric that takes into account the delivery delays of sensed data and the frequency at which a given region is sensed. The metric indicates, for a given time window, the proportion of streets that can be sensed under the requirements of an application. We apply the metric to more than 19 million GPS coordinates of 5,706 buses in Rio de Janeiro, mapping the coverage achieved for different application needs during a week. We build an abacus relating the coverage to different application requirements. We also calculate the coverage of a scenario only with static sensors. We show, among several observations, that bus-based sensing increases the coverage of applications by up to 7.6 times, in the worst case analyzed, when compared to a pure static scenario.