In this paper, we describe a variable fidelity simulation methodology we have devised and employed to develop, integrate and test low-level motion and perception as well as higher-level tactical vehicle behaviors for autonomous ground vehicles in a military operational environment. To enable autonomous vehicles to act as part of an infantry squad, performing similar tasks to those of junior squad members, we used two different simulation environments: 1) an in-house high-fidelity simulator for low-level mobility and perception, including local path planning; and 2) SimJr., a Java-based, low-fidelity, lightweight simulator for development and testing of high-level tactical vehicle behaviors, such as maintaining a formation within a fire team, or performing an independent reconnaissance task. Based on our experience, we provide recommendations on when the usage of the proposed methodology would be likely to save development time and cost, without sacrificing quality. Additionally, we examine advantages of system-wide logging and replay to capture inputs at the module boundary so that performance of individual components can be tested independently, to further reduce testing and debugging efforts. Finally, we describe how we employed a hardware-in-the-loop technique to test a tablet-based user interface with simulated vehicles and people to test UI functionality on a physical device without the effort and expense of deploying a vehicle.