The vehicle windshield defogging performance has direct influence under driver safety. In extreme situations, when the vehicle is not equipped with air conditioner and loaded with maximum occupancy, the defroster system must correctly direct the flow in order to defog pre-defined regions on the windshield. This process can't take too long and must guarantee a minimum fog-free area to increase driver's visibility.The main objective of this paper is to demonstrate the defogging simulation methodology developed by FIAT Brazil and Multicorpos Engineering. It uses CFD simulations with StarCCM+ software. Experimental data obtained from climatic chamber is used to compare the numerical data and validate the methodology.The simulations were able to conclude that defogging performance is correlated to flow homogeneity and its mean velocity on the internal windshield area. CFD simulations were used to indicate fin angles that can enhance the fog layer removal.