Abstract: A hydro-mechanical change speed mechanism of the type useful in road sweeping vehicles and the like in which the vehicle is required to travel from place to place at normal road speed but also requires the ability to travel at a relatively reduced speed while carrying out its main function such as road sweeping. The change speed mechanism herein disclosed is inserted into the normal drive train so that the input shaft 11 for the mechanism mates with the output shaft from a vehicle gear box and the output drive shaft from the mechanism 12 drives the driven wheels of the vehicle via a cardan shaft. A selector mechanism 41, 42, 43 enables the input and output shafts 11 and 12 to be mechanically directly connected together, or via respective first and second gear trains 15 and 16, to a vehicle drive pump 13 and a hydraulic motor 17 respectively. A hydraulic connection between the vehicle pump and the hydraulic motor results in an alternative hydraulic coupling between the input and output shafts 11 and 12.