Heavy-duty trucks are the major means of freight transport in India. After unloading the trucks, they are returned with empty loads to their destinations. Most of the aerodynamics studies on these trucks are focused on pickup truck tailgates, not heavy-duty trucks. As a result, the primary goal of this research is to examine India's most widely used heavy-duty vehicles (Taurus and Boss trucks). The study uses Computational Fluid Dynamics (CFD) to analyze the aerodynamics of on and off tailgates. The reliability and accuracy of the CFD simulation are validated with the literature. The flow characteristics of the heavy-duty trucks were examined in the cruise velocity of 14 m/s (50 kmph) corresponding to Reynolds numbers, Re = 1.87 × 106,  with respect to the tailgate on and off positions. The drag coefficient decreased by nearly 33.33% and 1.88% when the tailgate was off position for Boss and Taurus trucks. In the tailgate condition, a large reverse flow was observed in the wake zone, and an enlarged adverse pressure was found on the bed. Whereas in the tailgate off position, there were no recirculating flows in the wake zone; instead, a strong downwash was observed. The downwash promotes attached flow through the bed, which helps pressure recovery and reduces drag.