Numerous research efforts have focused on the development of tools for model reduction. Most of these efforts resulted in mathematical methods for model reduction which suffer from inadequate physical interpretations. The problem of simplified physical model design is just emerging as a major research area. Rare but interesting preliminary contributions address the model reduction issue from a physical point of view. However, they are more or less disconnected from the classical literature and tools dealing with model reduction. This paper considers the problem of simplifying physical models under the constraint of keeping the physical meaning. This is done in making use of the general principles of Bond Graphs together with the gramians as analysis tools. The method proposed is applied to the design of a low frequency vehicle driveline model.