This research project compares the in-use and laboratory-derived fuel economy of a medium-duty hybrid electric drivetrain with “engine off at idle” capability to a conventional drivetrain in a typical commercial package delivery application. Vehicles in this study included eleven model year 2010 Freightliner P100H hybrids that were placed in service at a United Parcel Service (UPS) facility in Minneapolis, Minn., during the first half of 2010. These hybrid vehicles were evaluated for 18 months against eleven model year 2010 Freightliner P100D diesels that were placed in service at the same facility a couple months after the hybrids. Both vehicle study groups use the same model year 2009 Cummins ISB 200 HP engine. The vehicles of interest were chosen by comparing the average daily mileage of the hybrid group to that of a similar size and usage diesel group. The driving characteristics of the two study groups were examined in detail by collecting and analyzing two periods of Global Positioning System and controller area network (CAN) bus data. The results of this drive cycle study indicated a need to have the two investigated groups switch route assignments mid-study because of significantly different driving patterns, which UPS accommodated, to facilitate an accurate comparison.The in-use fuel economy was evaluated using UPS's fueling and mileage records, periodic electronic control module image downloads, and J1939 CAN bus recordings during the two periods of duty cycle study. The drive cycle analysis was used to select three standardized laboratory drive cycles that would encompass the range of real world in-use data. The NYC Composite cycle, the HTUF Class 4 cycle, and the CARB HHDDT cycle were tested at the National Renewable Energy Laboratory's Renewable Fuels and Lubricants Laboratory. The hybrid vehicle demonstrated 39%, 45%, and 21% improvement in ton-mi./gal fuel economy on these test cycles, respectively. Analysis of the in-use fuel economy ranged from 13% to 29% hybrid advantage depending on measurement method and the associated delivery route assignment switch analysis showed 13% to 26% hybrid advantage on the less kinetically intense original diesel route assignments and 20% to 33% hybrid advantage on the more kinetically intense original hybrid route assignments.