Tamiya 3-Speed Crank-Axle Gearbox Kit
The Tamiya 70093 3-speed crank-axle gearbox is essentially half of the twin-motor gearbox.
The kit comes with enough parts so you can build the gearbox in any of three gear ratios (17:1, 58:1, and 204:1).
The output shafts included in this kit are 3 mm hexagonal axles that are 10 cm (about four inches) from tip to tip.
The low-voltage motors run on 3-6 volts and draw up to a few amps, making them perfect candidates for the Pololu Orangutan LV-168 robot controller (Product Code: 775), which incorporates these low-voltage motor drivers.