In-service emissions measurements were conducted on two package delivery trucks: one model year 2008 FCCC MT-55 conventional diesel and one model year 2012 FCCC MT-55 hydraulic hybrid (HHV). Mass emissions of CO2, CO, NOx, PM, and THC from the HHV and the conventional diesel test vehicle were each measured under conditions closely simulating normal package delivery operation.The HHV demonstrated a 29.4% improvement in fuel economy and a 17.4% reduction in CO2 emissions compared to the conventional diesel vehicle. The HHV showed its best potential in operating areas characterized by low driving speeds and high number of stops (“pick-up and delivery”) with a 40.5% improvement in fuel economy and a 21.2% reduction in CO2 emissions. Depending on operating conditions such as terrain, driver behavior and driving / traffic conditions, some fuel saving were achieved on operating areas characterized by high driving speeds and low number of stops (“highway / arterial”), but savings remained small.While the different EPA emission certification levels prevented the direct comparison of NOx emissions between the conventional diesel truck and the HHV, testing demonstrated that the HHV produced 62.6% less NOx per mile on the “highway / arterial” return than on the outbound trip. The analysis of the exhaust temperature of the HHV showed that this large difference in NOx emissions per mile was probably attributable to the low SCR catalyst temperature at the beginning of the trip.