Abstract: An automatic transmission system for a chain-driving vehicle includes a frame having a crank-driven sprocket and a drive axle with a drive wheel supported on the drive axle and a multi-speed sprocket assembly for driving the drive wheel. A continous chain drive engages the crank-driven sprocket and is engageable with each of the sprockets of the sprocket assembly. An actuating means is supported on the drive wheel spaced from the drive axle and is actuated by centrifugal force for shifting the chain to respective sprockets of the sprocket assembly in response to changes in speed of the vehicle.