Underhood cooling systems are crucial for ensuring efficient thermal management leading to improved vehicle performance in terms of range and thermal derating of key powertrain components, while meeting packaging constraints. This work presents a synergetic 1D-3D solution by modeling external air flow with 3D computational fluid dynamics (CFD) and internal flow (coolant, refrigerant and gas) with 1D CFD in GT-SUITE. The presented methodology helps in achieving higher computational speed owing to 1D-2D spatial discretization of flow inside the heat exchangers and high accuracy due to detailed 3D CFD for external air flow. Moreover, the iterative exchange of boundary data between 1D and 3D domain ensures solution accuracy and stability while reducing computational efforts. Accordingly, a test case with inclined cooling module is presented, where the design optimization focuses on key parameters such as degree of inclination of different heat exchangers (HXs), HX dimensions, flow space geometry, fan characteristics and grill design. Optimization techniques like design of experiments and multi-objective Pareto optimization are deployed to balance cooling efficiency and energy consumption within given system constraints. In conclusion, this coupled 1D-3D modeling approach provides a unique numerical solution that is suitable for performing high number of design iterations desired for pre-design of vehicle cooling systems to make faster and accurate decisions.