Currently, car bodies require further weight reduction in order to support increasing fuel economy requirements. An efficient way for light weight body design is to include body member size as a design variable in addition to part thickness. However it is currently difficult for finite element (FE) models to change member size even using current morphing techniques. To break through this challenge, a hybrid modeling approach was developed which combines shell and beam element representations of body structural members. The original member shell element thickness was decreased by 40%. Then the stiffness reduction caused by this change is offset by beam elements incorporated inside these members. These beams can represent the stiffness change due to new cross sectional dimensions or orientations without changing the original shell elements, thus avoiding modeling instabilities that can occur from morphing. This idea allows greater design freedom for the members without compromising the analysis accuracy or capability of the original mesh model. Optimization for weight reduction while maintaining body rigidity was carried out for a mass production car model with Genetic Algorithms (GA) . A weight reduction of 10 kg was achieved without any reduction of body rigidity. A unique benefit of this proposed hybrid modeling technique is that it can be used for acoustic analysis. This is because the original shell elements required for the coupling between the fluid and structure are unchanged. An optimization with GA was performed in order to reduce acoustic sensitivity and weight. The acoustic sensitivity around 50 Hz was reduced by 6 dB with a weight reduction of 7.5 kg. Therefore, the benefit of using the new hybrid modeling technique for optimizing member size was validated.