Gearing

Unless you are using a motor with a gear head or a very strong and high resolution stepper motor, you probably need to gear the motor down to get the right speed and torque.

Gearing beyond 5:1 requires multiple gears. Although this can be done, it is generally difficult to be done right. As a result, you should select the motor properly so that you do not need to gear it down too much.

For a low gearing ratio, you can pretty much use any type of gear. Plastic gears tend to have less grinding and are generally less expensive.

One option is not to use gears at all. Instead, have the drive axle of the motor directly driving the wheel via friction. See the following figure.

Figure 3.1: Direction Friction Drive

$$\includegraphics{friction}$$

In order for this approach to work, you need the tire to have good grip, and you need some force pushing the motor axle against the wheel.