A mainshaft assembly for a gearbox and a gearbox embodying such a mainshaft assembly are disclosed. The mainshaft assembly comprises a mainshaft (44), and several drive gears (31 . . . 36), each carried for rotation about the mainshaft, each drive gear having a different number of teeth. The assembly includes first and a second hub (50, 50′), each being associated with a respective drive gear, each hub having a dog ring (54, 54′) operable to selectively couple with the drive gear causing it to rotate with the hub or uncouple from the drive gear to allow the drive gear to rotate with respect to the hub. A respective drive connection including a cage (42) and pawls (64, 66) is associated with each hub and selectively connects the hub to the mainshaft for rotation with it or to allow rotation with respect to it when the pawls are displaced against a spring bias. Upon connection of both first and second hubs by their respective dog rings to each corresponding drive gear, the drive connection operates to connect one of the hubs to the mainshaft when torque is applied to the mainshaft in a first direction and to connect the other one of the hubs to the mainshaft when torque is applied to the mainshaft in the opposite direction. This enables selection of a gear ratio provided by one or other drive gear to be achieved by reversal of the torque being handled by the gearbox (as happens, for example, when a vehicle goes through a transition from accelerative drive to coasting). There is no delay while drive engagement means take up or disengage drive.