Those are gear motors. It is a good way of propelling a tank as long as you can calculate the maximum RPM you would need because that is how these are made by RPM of the gear reduction at the pinion gear.Or end of the sprocket shaft. Then you need to fix the sprocket to the reduction output shaft.<div>http://www.servocity.com/html/3-12v_gear_motors.html</div><div> These are examples of gear motors.</div>