A method of controlling a propulsion system of a work or agricultural vehicle, with hydraulic transmission adapted to work according to a first operating mode (ST1) in which an inclination of an accelerator lever proportionally determines a rotation speed of the prime mover and a second operating mode (ST2) in which said lever inclination indicates a vehicle speed, while the prime mover is operated at a predetermined approximately fixed operating point, the method, in said first operating mode, comprising a first step (Step 1) of verification that a vehicle speed is approximately constant for a predetermined time interval, then when the verification is positive (yes), a second step (Step 2) in which the propulsion system is automatically controlled to reduce the rotation speed of the prime mover moving to said predetermined operating point and to adapt the hydraulic transmission so as to maintain the vehicle speed was unchanged during the reduction of the rotation speed of the prime mover.