A 13 degrees of freedom (DOFs) model was established that includes a powertrain mounting system (PMS), a body, and four unsprung masses. In the model, the mounts in the PMS and the tires were simplified as stiffness and damping elements. The degrees for the powertrain, body and unsprung mass were 6, 3 and 4, respectively. Some widely used models were specific cases of the presented model of 13 DOFs, such as the 6 DOFs model (powertrain′s 6DOFs) for the design calculation of a PMS, the 7 DOFs model (body′s 3 DOFs and unsprung mass′s 4 DOFs) and the 9 DOFs model (powertrain′s 6 DOFs and body′s 3 DOFs) for ride analyses of a vehicle. The natural frequencies and modal energies for the powertrain in a PMS were calculated, and the results were compared and analyzed using different models. A difference in the calculated frequency and energy of the powertrain in the bounce direction was demonstrated using the mode with 6 DOFs compared to other models with 9 and 13 DOFs. The model with 6 DOFs was simple and required fewer input parameters for calculations, so it is used widely in the initial design stage of a car. The models with 7 DOFs can be used in the calculation of natural frequencies and mode energies for the body and unsprung mass.