Abstract: The Australian Army is procuring field vehicles incorporating new in-vehicle information technologies to increase their logistics support capacities in high-risk areas. These technologies will require multi-person crews to replace single driver-operators. As part of a research program exploring the impact of performance-shaping factors on vehicle crews, 16 soldiers operated simulated vehicles in driver/co-driver pairs, under different levels of task demand and over extended time periods. Participants performed a primary task related to their crew role, along with a concurrent auditory secondary task which was varied in complexity and incorporated a dyadic within-crew communication component. Co-drivers, whose primary task was a continuous N-back visual working memory task, experienced significant performance declines with time-on-task across all tasks, with greater deterioration when secondary task complexity was lowest. Drivers, whose primary task was maintaining consistent speed and lane position, showed relatively stable performance across tasks, regardless of secondary task complexity. The results provide support for effort regulation theory, with potential under-mobilisation of effort and resources to compensate for underload effects (such as loss of task-related engagement) induced by monotonous, undemanding conditions. Potential strategies to counter such conditions in field vehicles (e.g., secondary tasks, in-vehicle systems), study limitations, and directions for planned research are discussed.