The global automotive industry is moving towards sustainable mobility through vehicle electrification & autonomous connected vehicles. The vehicle electrification trend has just begun with Electric Vehicles (EV) accounting for only ~3% of sales worldwide, and expected to increase considerably in the near future. OEMs face many challenges in this relatively new field, starting from manufacturing challenges such as material availability for battery and electric components, as well as availability of charging infrastructure, to vehicle thermal challenges such as battery range and thermal comfort. In extreme weather conditions specifically, the lack of waste heat from the internal combustion engine (ICE), and high resulting energy expenditure of the climate system, lead to a constant trade-off between occupant’s thermal comfort & battery range. Even with the recent proliferation of EVs, range anxiety still exists; in extreme winter weather conditions, this range can reduce by more than 20%, losing power to cabin heaters, seat heaters and defrosters. It is important to strike a balance between range and comfort to design an energy efficient Electric Vehicle, and in an environment ripe with competition, and privileging fast development methods, simulation representing realistic usage scenarios can be very beneficial. An advanced virtual Twin can help to size the Heating, Ventilation & Air Conditioning (HVAC) system, and predict battery life, vehicle range and thermal comfort during early design stages. In this study, a virtual co-simulation of the system level & 3D CFD model is used to capture complex interactions across scales, in order to optimize overall vehicle performance. Co-simulation capabilities involving 3D CFD and FEA (Finite Element Analysis) thermal models provide high fidelity predictions of passenger thermal sensation and battery temperature distribution. Further integration with Dymola system behavior models enables the prediction of real-world driving scenarios for all systems in the vehicle. Additional 3D CFD analysis for the underhood airflow is also performed, to characterize the external air received by the front-end heat exchangers of the HVAC system in different vehicle operation conditions. The airflow characteristic is used in the 1D HVAC system analysis to provide realistic boundary condition for the heat exchangers. In addition, this co-simulation approach, the study also presents a methodology for efficient sizing the HVAC by leveraging reduced order modelling. A set of accurate & detailed cabin comfort thermal results from CFD are aggregated and mapped to define the cabin performance in the system model. With this approach, the vehicle performance can be quickly optimized with multiple quick system model simulations, benefiting from trends captured in detailed 3D CFD response surfaces.