Abstract: A differential gear drive having two output shafts each coupled through a differential gear train to a common input gear. The differential gear train comprises an annulus gear engaging the input gear, planet gears and a sun wheel. The output shaft is connected to a planet carrier and the output shaft is connected to the sun wheel. A pair of cam rings are rotatably coupled to the respective output shafts and are respectively coupled via clutch teeth to clutch rings respectively, which are engageable separately with a pair of meshing reference gears. The reference gear is rotatably fixed by a countershaft to the input gear. When both output shafts are driving against substantially equal torques the clutch rings are both disengaged from the reference gear and so the drive to each shaft is via the differential gear train.