A mathematical model of an automotive transmission is developed, that considers the flexibility of the shafts, bearings, and gear teeth, and gyroscopic effects of geared rotors. The transverse, torsional, and axial motions are strongly coupled due to helical gearing. The excitation forces acting on the automotive transmission are classified into first, second, and third grades, based on the magnitudes of the forces that are determined by the perturbation method. The excitation forces are caused by the mass imbalances among gears, misalignment of shafts, clearance and non-linear deformation of bearings, transmission errors, and the periodic variation of the gear mesh stiffness. A bench test on loading conditions is carried out for the third speed of the automotive transmission. The experimental results of vibration characteristics are compared with those from theoretical analysis. The results show good agreement, i.e., within a tolerance of 3.3%.