The Hybrid Electric Vehicle Fuel Economy Methodology Study was conducted by the Automotive Instrumentation Division, US Army Aberdeen Test Center (ATC), Aberdeen Proving Ground (APG), Maryland, from June 2006 through August 2009. The program objectives were to develop a test protocol that can be used to evaluate the fuel consumption characteristics of a hybrid electric vehicle regardless of weight class, battery chemistry, and/or driveline configuration, and to characterize the performance of currently developed hybrid vehicles and tactical wheeled vehicle prototypes with regard to fuel consumption and energy usage. Eleven hybrids and eight conventional vehicles were provided for the methodology study. Fuel consumption tests were conducted on a wide spectrum of terrains ranging from level paved road surfaces to hilly cross country secondary road surfaces. Test vehicles were operated over the full range of speed capabilities on each of the terrain scenarios. Results for ground-up or conversion hybrid vehicle designs were compared to conventional vehicles of similar test weight or of the original chassis design in the case of conventional vehicles converted to hybrid propulsion. In accordance with a Design of Experiment (DOE) established by US Army Tank Automotive Research Development and Engineering Center (TARDEC), individual test trials were conducted with conventional and hybrid comparison vehicles in a leader-follower arrangement. Electrical energy storage system State of Charge (SOC) correction methods were evaluated and established to present hybrid vehicle fuel economy and consumption by correcting fuel measurements to a ΔSOC=ZERO fuel economy. Candidate vehicles were also characterized for power loss, weight distribution, center of gravity, and selected automotive performance capabilities.