The gearbox has an idler shaft which is coupled with the vehicle drive shaft and carries an idler gear (6). It has inward-facing teeth (1a) engaging with matching teeth on a main wheel (1) running on a two- row roller bearing (3) on a main shaft (5). The rollers are arranged in a cage (4) mounted on a distance ring (9). The synchromesh arrangement (8) for engaging the gear incorporates a ring (81) which may be moved sideways on a hub (83) and brings a synchronising ring (84) into contact with a friction cone forming part of a coupling wheel (7) with teeth which can engage on teeth on the ring.
