Abstract: In a power transmission comprising a driving shaft and a driven shaft coupled through a clutch unit, the rotational velocities of the shafts are measured by two sensors mounted on the declutching member which slides on the driven shaft to actuate the clutch diaphragm. The sensors may for example be of the variable reluctance type, and are positioned so as to face a portion of the driven shaft and another part of the driving shaft which comprise at least one element or structural feature to which the sensors are responsive, such as ribs or grooves constituting a kind of "pinion" of the driven shaft and a finger which is fixed with respect to the cover plate of the clutch, the latter being itself rotatable with the driving shaft. This "pinion" may be modified so as to allow the degree of wear of the clutch friction pads to be measured.