Abstract: A vehicle in which a prime mover drives a drive shaft via a torque converter and a multi-ratio transmission and in which a drive line independent of the torque converter and multi-ratio transmission is also provided from the prime mover to the drive shaft. The independent drive line provides an overdrive or underdrive function to extend the drive ratio range provided by the multi-ratio transmission and includes a clutch for the engagement of the additional ratio or ratios, and an interlock arrangement to prevent the simultaneous connection of drive to the drive shaft via the multi-ratio transmission and the independent drive line. When an overdrive function is provided, for example the interlock arrangement prevents engagement of the overdrive ratio unless the multi-ratio transmission is operating in its highest ratio when engagement of the overdrive is selected.