if you study it closely. you'll notice that can be expanded beyond 10 speeds by adding another pair of extensions and then four reguler transmission driving rings. You can continue to expand it beyond this point by again adding two extension driving ringa and so on. There's no limit to how many speeds can be added this way, except that the number of dead gears in the transmission increases quickly and a 14-speed version generates enough resistance to stall a PF XL motor.