Tamiya’s twin-motor gearbox consists of two independent brushed DC motors and gearboxes in one compact housing, making it perfect for small robots. You can assemble the gearboxes to have a 58:1 or a 204:1 gear ratio.
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.