In this paper, considering a half car model with 3-DOF, the effects of vehicle velocity, wheelbase, and the vibration parameters of the passenger seat on the statistical characteristics of the vehicle body and the seat responses are investigated. In the previous studies, 2-DOF model was generally used for vibration analysis of the vehicle under the road random excitation. However, science these models ignore the DOF corresponding to the seat, they are not suitable for ride comfort analysis. Here, in order to overcome this shortage and make it possible to study the ride comfort, considering an additional DOF for the seat, a half car 3-DOF model has been used. Moreover, in order to investigate the Root Mean Square Acceleration Response (RMSAR) of the vehicle body and the seat, a mathematical relation is proposed for the power spectral density (PSD) of the actual highway- type road excitation. Finally, by applying the proposed PSD, the ride comfort of a special passenger car (Samand) has been studied. The results show that the range of variations of RMSAR of the vehicle body and that of the seat are completely different from each other, and by choosing reasonable values for the stiffness and damping coefficients of the seat, one can improve the ride comfort.