Pololu 150:1 Micro Metal Gearmotor

This Pololu Micro Gear Motor HP has a long 9 mm, D-shaped metal output shaft that matches the Solarbotics RW2 - Large Rubber Wheel and the Pololu Wheel 42x19 mm (Pair) . The brass faceplate has two mounting holes threaded for M1.6 screws (1.6 mm diameter, 0.35 mm thread pitch), or you can use Pololu's Pololu Micro Metal Gearmotor Bracket (pair) or Pololu Micro Metal Gearmotor Mounting Bracket (Extended-Pair) , which are specifically designed to securely mount the gearmotor while enclosing the exposed gears.

These motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 - 9 V range. Lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor.