Abstract: An automatically variable speed ratio transmission for a pedal driven vehicle such as a bicycle with a chain and sprockets. A pedal operated chain driving sprocket is capable of having its diameter infinitely variable through a predetermined range. The driving sprocket consists of a plurality of small sprockets journally mounted on bell-crank arms having planar offsets for rotation of the small sprockets in one direction only. The bell-crank arms are pivotally mounted on a disc in a uniform circular pattern therearound. The arms and their small sprockets are arranged and constructed to define the vehicle's driving sprocket. The plurality of bell-crank arms are interconnected with each other and are arranged to be uniformly spring biased in one direction.