Tractor-trailer units are integral part of the heavy commercial vehicle 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-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. Keywords Heavy commercial vehicle (HCV) • Computational fluid dynamics (CFD) • Computer-aided design (CAD) • Computer-aided engineering (CAE) • Aerodynamic coefficients List of symbols C D Drag coefficient vehicle (-) C L Lift coefficient of the vehicle (-) C S Side coefficient of the vehicle (-) C P Coefficient of pressure (-) l Length of the vehicle (m) h Height of the vehicle (m) w Width of the vehicle (m)