This paper investigates the effect of the powertrain mounting system on the linear and nonlinear torsional dynamical behaviour of a transmission system. To this aim, two dynamic models, one with rigid mounts and the other with flexible mounts, are presented and compared: the first model considers only the torsional dynamics of transmission and driveline, while the second model includes also a 3 degrees-of-freedom powertrain block. The mechanical coupling and interaction between the powertrain block and transmission system is discussed and formulated. These models are then analyzed in terms of vibrational mode shapes, natural frequencies and Frequency Response Functions (FRFs); a sensitivity analysis of the main transmission parameters, e.g. the gear ratio, is also presented. From the comparison of the two simulated configurations (with and without powertrain mounts) both in time and frequency domain, a significant interaction between the two subsystems is noticed, particularly in the vehicle acceleration signal. Neglecting the powertrain mounts may lead to underestimating the real vibration level. A nonlinear model of a dual clutch transmission, including the main backlashes present along the transmission path (clutch spline, gear meshes, synchronizers), is coupled to the powertrain mounting system model. Finally, the effect of mount stiffness on transmission NVH during critical manoeuvres is shown.