Tamiya’s 3-speed crank-axle gearbox looks very much like half of the twin-motor gearbox (with an added gear ratio option). This compact gearbox can be built in one of three gear ratios (17:1, 58:1, and 204:1), and is Tamiya’s cheapest, general-purpose gearbox.
Documentation and other information
Tamiya Gearbox Gear Ratio Comparison
Table comparing the gear ratios available in the many different Tamiya gearboxes we carry.
Tamiya “Toy” Motor Testing by Adam Borrell (a.k.a nexisnet) (Printable PDF: tamiya_toy_testing.pdf)
Adam, who posts under the username nexisnet on the Pololu forums, has performed a detailed series of experiments to determine how operating voltage affects the lifetime of the Mabuchi FA-130 motor, which happens to be the motors used in the Tamiya 70168 Double Gearbox, 70167 Single Gearbox (4-Speed), 70097 Twin-Motor Gearbox, 70093 3-Speed Crank-Axle Gearbox, 70110 4-Speed Crank-Axle Gearbox, and 70103 Universal Gearbox.