The upshift ratios of a gearbox from ratio i to ratio i+1 in traction comprise the following steps: applying a slight force of disconnection on the idling gear of the initial ratio i; gradually drifting the engine torque towards the idling gear of a ratio n higher than i; by clamping a friction member (23) with controlled pressure, disconnecting the first idling gear; synchronising the engine and the input shaft by adjusting the clamping pressure of the friction member, while transmitting the desired torque; detecting the oncoming speed synchronism on the new ratio i+1; disconnecting the idling gear of the new ratio i+1; and gradually releasing the friction member.