Abstract: A particularly compact planetary gear mechanism includes an axially shiftable ring gear to provide either a direct drive mode or a speed reduction ratio mode between an input sun gear thereof and an output planet carrier thereof. In a first embodiment, the ring gear is urged in a first direction by a compressible spring apparatus to a conjoint coupled relationship with the sun gear for direct drive. In a less frequently used mode the ring gear is urged by a hydraulically actuated annular piston closely encircling it into a fixed relationship with a stationary coupling in opposition to the compressible spring apparatus. A second embodiment has no spring apparatus so that the annular piston is hydraulically biased in either axial direction for positively shifting the ring gear.