In this paper, a theoretical approach is studied to predict structural performances and weight reduction rates of a car-body with shell type sections in case that its materials have to be substituted. For the material substitution design of a car-body, bending, axial and twisting deformations are considered under constant stiffness and strength conditions, which utilize some new indices derived from a structural performance point of view. The developed indices to measure the weight reduction by the material substitution give good guidelines on conceptual design of car-bodies.