In this paper we present the results of full-scale chassis dynamometer testing of two hybrid transit bus configurations, parallel and series and, in addition, quantify the impact of air conditioning. We also study the impact of using an electrically controlled cooling fan. The main trend that is noted, and perhaps expected, is that a significant fuel penalty is encountered during operation with air conditioning, ranging from 17-27% for the four buses considered. The testing shows that the series hybrid architecture is more efficient than the parallel hybrid in improving fuel economy during urban, low speed stop and go transit bus applications. In addition, smart cooling systems, such as the electrically controlled cooling fan can show a fuel economy benefit especially during high AC (or other increased engine load) conditions. The series hybrid bus was equipped with an active Diesel Particulate Filter (DPF); the filter was found to be in active mode during the AC-on tests, which adversely impacted the fuel economy. The DPF operation prevented a direct comparison of the series and parallel hybrids with AC-on. Some interesting features of the active DPF were noted during operation; these included an increase in hydrocarbon emissions during active regeneration with the main hydrocarbon being methane. In contrast, unburned diesel fuel was the primary hydrocarbon constituent during standard operation (i.e., passive DPF or DOC).