A belt drive system employs variable diameter drive and driven pulleys interconnected by a flat belt and operative to vary the rotational speed of the driven pulley as in a variable speed transmission. Each pulley includes an internal fluid pressure chamber and diametrically aligned pairs of radially movable pistons or lift bars that support transverse lift rods at their outer ends. Axially spaced circular guide plates form end boundaries for the fluid pressure chamber and have radial through-slots that cooperate with arcuate slots in juxtaposed pairs of coaxial guide discs to receive opposite ends of the lift rods and effect synchronous radial movement of the lift rods in response to pressure changes within the fluid pressure chamber. An elongated flexible spring-like support member is attached at one end to a lift rod and extends in supported relation circumferentially over and about the lift rods to define a variable diameter belt support surface. The diametrically aligned pistons are adapted for telescoping relation during radial inward movement of the lift rods, thereby enabling greater variance in the diameter of the belt support surface for a given size pulley. Electronic sensors sense the rotational position of each pulley during rotation and cooperate with a source of fluid pressure to maintain the desired pulley diameter and belt tension.