An improved gear change mechanism is provided for semi-automatically selecting a change up or change down gear change. The mechanism includes a shaft (31) which is movable in its axial direction and about its axis to make one such movement or a combination of such movements according to the gear to be selected. The shaft (31) is moved by pistons (36, 37) coaxial with the shaft for the axial movement and a piston (46) at a right angle to the shaft for the rotational movement. In practice the axial movement is a translating movement and the rotary movement is a movement into and out of engagement with the gears. Throttle and clutch control functions are also provided so that a gear change is initiated and brought about by switches (19A and 19B).