A method is provided for controlling the position of a gearbox actuator in charge of engaging a ratio at the end of a preliminary phase of synchronizing two shafts of the gearbox via a torque-driven traction machine to bring the speed difference of the two shafts within a range enabling the mechanical coupling thereof. The method ensures that the speed measured on one of the two shafts converges with a speed observed as a function of the inertia value observed on this shaft relative to the value expected as a function of the gearbox actuator, and of an estimation of the machine torque.