Tractor-trailer units are integral part of the Heavy Commercial Vehicle (HCV) industry, used globally for goods transportation. Manufacturers have been trying to design aerodynamically efficient tractor-trailer units to reduce ever increasing fuel costs. In order to investigate the aerodynamic response of tractor-trailer units, the aerodynamic forces and moments have to be determined accurately, especially under crosswind conditions. In the present study, a Computational Fluid Dynamics (CFD) based solver has been employed to simulate the flow field around a tractor-trailer with a view to quantify the effects of side wind and size variations on aerodynamic force moment system acting on tractor-trailer combination. It has been shown that the aerodynamic forces are significantly influenced by both the geometrical and flow characteristics. The drag, lift and side forces acting on a tractor-trailer unit are highest at relative flow angles of 15°, 30° and 90° respectively. Aerodynamic forces and coefficients have been enumerated for these geometrical and flow conditions, and have been used to develop novel semi-empirical correlations for the aerodynamic coefficients for the tractor-trailer unit. These correlations have been shown to predict the aerodynamic coefficients for various vehicle dimensions under a range of flow conditions with reasonable accuracy.