Integrated simulation capabilities that are high-fidelity, fast, and have scalable architecture are essential to support autonomous vehicle design and performance assessment for the U.S. Army's growing use of unmanned ground vehicles (UGV). The HMMWV simulation described in this paper embodies key features of the real vehicle, including a complex suspension and steering dynamics, wheel-soil models, navigation, and control. This research uses advanced multibody techniques such as minimal coordinate representations with constraint embedding to model complex unmanned ground vehicles for fast mechanical simulations with high fidelity. In this work, we demonstrate high-fidelity dynamics models for autonomous UGV simulations in near real time that can be useful to the U.S. Army for future autonomous ground vehicle dynamics modeling and analysis research.