Abstract: A stepless variable ratio transmission is provided having an input drive axle connected to a sun gear of a planetary gear assembly, an output axle connected to a ring gear of the planetary gear assembly, and a planet or pinion carrier rotatably supporting a plurality of planet or pinion gears thereon, the pinion gears being meshingly engaged with the sun gear and the ring gear. A control means is also provided for controlling the rotational speed of the planet carrier, the control means comprising a worm gear driven by a servo or stepping motor, which is in turn controlled by a computer which senses various conditions in determining the desired transmission ratio. The planet carrier is provided with a plurality of gear teeth along a peripheral edge thereof to meshingly engage the worm gear. The design of the worm gear and the planet carrier is advantageously selected to improve the efficiency of the system by minimizing the power required to effect the rotational speed control of the planet carrier.