However, they aren't very fast. The gears bring the main axle speed down quite a bit. Once I modifed the servos for full rotation I measured the rotation rate to be slightly less than one rotation per second. If I round this to one rotation per second then it is easy to find the speed of the robot for a given wheel size. The speed is simply the circumference of the wheel.
CDs do have the advantage of being very lightweight which is nice.
I am a little concerned that they may slip badly on smooth floors. If this turns out to be the case I will either stretch a rubber band around the edge of the CDs or I will cut, file, or sand a rough edge onto the CDs. The main body is a wooden box approximately 6.5 inches in diameter and 2.5 inches deep. The wheels won't be mounted on the sides of the box. Instead I am cutting parallel slits through the top and bottom of the box as far away from each other as possible while still being long enough to enclose the CDs' diameters.