A transmission unit particularly for driving screw feeders and auxiliary user devices of mixing trucks, comprising an off-center distribution device, which can be associated with an external power take-off and forms couplings for the connection of auxiliary user devices; a reduction unit, which is functionally connected to the off-center distribution device and can be associated with the screw feeders of a mixing truck; and a main friction clutch device, which is interposed between the off-center distribution device and the reduction unit, a device for reversing the direction of rotation of the screw feeders and an auxiliary friction clutch device for the reversing device.