We propose a composite thermal model of the vehicle passenger compartment that can be used to predict and analyze thermal comfort of the occupants of a vehicle. Physical model is developed using heat flow in and out of the passenger compartment space, comprised of glasses, roof, seats, dashboard, etc. Use of a model under a wide variety of test conditions have shown high sensitivity of compartment air temperature to changes in the outside air temperature, solar heat load, temperature and mass flow of duct outlet air from the climate control system of a vehicle. Use of this model has subsequently reduced empiricism and extensive experimental tests for design and tuning of the automatic climate control system. Simulation of the model allowed several changes to the designs well before the prototype hardware is available. In addition to the reduced vehicle field tests and wind tunnel tests man hours and the cost associated with them, simulation of the model allowed for the greater potential benefits of increased accuracy and optimized heating and cooling of the passenger compartment to be achieved. An outlook is provided for integrating the composite thermal model of the passenger compartment with the human thermal comfort model to achieve greater benefit of optimized energy use by the climate control system and yet provide the thermal comfort to the occupants.