Truck-trailers are required to have rear impact protection guards per Federal Motor Vehicle Safety Standards 223 and 224. The standards define the minimum strength and energy absorption requirements at the guard component level, while allowing the guard manufacturer to use a rigid test fixture when certifying the guard. Due to the limitations inherent in “rigidizing” the under-structure of a trailer, often some amount of deformation in the supporting structures is tolerated when certifying a guard. Hence there is a tendency to certify the impact guard as a “guard system” composed of guard members and the support (mounting) structures. In this paper, a previously validated 1990 Ford Taurus FE model was used to analyze the effect of compliance in the guard support members on its dynamic performance. Two guard systems, one with rigid supports and another with some compliance in the supports were modeled. The two guard systems had the same strength in accordance with the requirements of FMVSS 223. The underride collisions were simulated at various speeds to compare and contrast the performance of the two guard systems. Observations were made as to why the amount of underride was substantially different for the two guard systems.