Pololu 9.7:1 Metal Gearmotor 25Dx48L mm HP 12V

This gearmotor consists of a high-power, 12 V brushed DC motor combined with a 9.68:1 metal spur gearbox. The gearmotor is cylindrical, with a diameter just under 25 mm, and the D-shaped output shaft is 4 mm in diameter and extends 12.5 mm from the face plate of the gearbox. This gearmotor is also available with an integrated encoder.

In general, these kinds of motors can run at voltages above and below their nominal voltages (they can begin rotating at voltages as low as 1 V); lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor.