Abstract: A gearbox comprising two shafts (20, 27) with their axes parallel to one another and at least two pairs of gear wheels each pair comprising a wheel (38) on one of the shafts permanently in mesh with a wheel (45) on the other shaft, the wheels (45) on one of the shafts being connectable to the shaft by friction clutches, and actuating means disposed between adjacent friction clutches to selectively engage one or other of them; the actuating means comprising two pressure rings (2) and an adjusting ring (1) disposed between them, adjusting rings (5) for causing relative angular movement between the adjusting ring and pressure rings, and opposed pairs of grooves (21, 22) in the facing surfaces of the adjusting ring and pressure rings.