Transmission synchronized speeds

The invention concerns a motor vehicle gearbox (10), comprising at least two parallel primary (14) and secondary (16) shafts, respectively connected in rotation to an engine and, through a main transmission (20), to a wheel (18), and wherein the primary shaft bears a first idler (22) geared to a second stationary pinion (24) borne by the secondary shaft, which first idler (22) is capable of being jaw clutched on the primary shaft for transmitting engine power to the wheel (18). The invention is characterised in that it comprises self-disengaging controlled means (28) for accelerating or braking selectively the primary shaft (14) which bears the idler (22) so as to synchronise them before linking them in rotation, self-disengaging acceleration means for accelerating the primary shaft (14) as long as its speed is less than that of the idler (22), and self-disengaging braking means for braking the primary shaft (14) as long as its speed is greater then that of the idler (22).