Abstract: A hydraulic transmission device comprises a rotatable casing secured at one end by a first flange to an input shaft and at the other end to a second flange which supports an output shaft. Drive is transmitted by fluid introduced into an area between the outwardly directed contoured teeth extending around the periphery of a rotor, eccentrically mounted on said first flange for rotation therewith, and the inwardly directed contoured teeth extending around the periphery of an annular ring secured to said output shaft, the diameter of the rotor being smaller than the internal diameter of the annular ring thereby allowing the respective teeth of the rotor and the ring to mesh during rotation of the input shaft.