A comprehensive research is presented aiming at assessing the ride comfort of subjects seated into road or off-road vehicles. Although many papers and books have appeared in the literature, many issues on ride comfort are still to be understood, in particular, the paper investigates the mutual effects of the posture and the vibration caused mostly from road unevenness. The paper is divided into two parts. In the first part, a mathematical model of a seated subject is validated by means of actual measurements on human subjects riding on a car. Such measurements refer to the accelerations acting at the subject/seat interface (vertical acceleration at the seat cushion and horizontal acceleration at the seat back). A proper dummy is used to derive the seat stiffness and damping. An in-depth theoretical analysis is performed to assess how random vibrations may affect the validation of the mentioned mathematical model having five degrees of freedom (head, torso, legs and feet) and 30 parameters. In the second part of the paper a sensitivity analysis is performed based on the validated mathematical model. A number of different ride comfort indices (SEAT, K (VDI), VDV, …) are accounted for. A number of different roads are considered. The parameters referring to the posture seem the ones influencing mostly the ride comfort, together with the seat cushion damping. Other parameters have less important effects.