A transmission is disclosed for trucks of the type having a wheeled chassis that is towed or independently powered and supports a substantially inverted frustum-shaped container which is upwardly open and contains a scroll with a vertical axis for shredding and mixing fibrous products. The transmission comprises a tracked-type epicyclic reduction unit which is mounted on the bottom of the container and supports the scroll in a cantilevered manner. The reduction unit is coupled to a hydraulic motor connected to a pump, which is in turn coupled to a drive unit.