Abstract: A transmission system for vehicles. The transmission system has a pulley mounting a shaft extending from a mandrel of a power source, an automatic clutch for driving a wheel and a belt linking the pulley with the clutch. The pulley has a first section securely mounting the shaft, a second section axially moving along the shaft and a disk securely mounting the shaft. The first and second sections each have a belt-engaging side shaped as a truncated cone. The second section further has a slide-engaging side formed with a number of radial slopes. A corresponding number of slides contact the slopes and the disk.