Abstract: A gearbox adaptor (2) which can be combined with a standard vehicle gearbox to convert a standard gearbox to a sequential gearbox. Some or all of the gears (5, 6) of a standard gearbox have the synchro-hubs and cones removed and one or more gearbox adaptors (2) substituted. Each adaptor (2) includes a hub (4) which engages the gear shaft (3), at least one piston (7, 8) mounted within the hub (4), means for supplying fluid (16) from the exterior of the hub (4) to a first face of the or each piston (7, 8), at least one clutch means (9, 10), part of which engages the hub and another part of which engages a gear (5, 6) located on the gear shaft (3); the arrangement being such that movement of the piston (7, 8) in a predetermined first direction inter-engages the parts of the clutch (9, 10) to drivingly engage the gear (5, 6) and the gear shaft (3).