To obtain maximum vehicle performance without causing in a power train system for vehicle a damage and excessive wear to a drive train system.

A power train system 10 is controlled by an output torque of an engine 12 as a function of an engagement gear ratio of a transmission 14. Different torque limit values A, B, C, and D are determined for from a take-off gear ratio, midpoint gear ratio, direct-coupled gear ratio and overdrive gear ratio respectively. Further, the torque limit values A, B, C and D satisfy relations of A< B<C> D and B<D. This enables maximum vehicle performance without causing a damage and excessive wear to a drive train.