The fictitious-mass modal coupling method is extended to include sensitivity analysis of system-level performance to parametric changes in sub-system structural properties. The resulting model is very efficient, of high-accuracy and facilitates extensive parametric design sessions in an on-line manner using common utility software packages. The extended method is demonstrated by the evaluation of acceleration response parameters of a typical vehicle traveling over a rough road, and their derivatives with respect to local structural variations. The vehicle is divided into several structural components represented by separate normal modes produced with fictitious masses loading their interface coordinates. The examined variables are stiffness and damping values of component interconnection elements, and natural frequencies of the separate components.