The optimum gear ratio for a particular robot depends on many factors. Three primary parameters are the size of the wheels, the desired robot velocity, and the rotational speed of the motor. The following table relates these parameters in a simple way. The table assumes a desired robot velocity of 6 inches per second and a motor rotational speed of 5,000 rpm.

Wheels	Diameter, in.	Inches/rev.	Wheel rpm	Gear Ratio
Large LEGO Wheels	2.5	7.9	45.8	109
Small LEGO Wheels	1.9	6.1	59.1	85
Large Tires	1.6	5.1	70.3	71
Medium Tires	1	3.1	114	44
Monster Truck Tires	3.1	9.6	37.4	134
Cart Wheels	5	15.6	22.9	218

The third column gives the circumference of the wheel, or the inches that the robot travels per revolution of the wheel. The fourth column indicates how many revolutions per minute the wheel must turn to achieve a velocity of 1 ft/sec, and the last column indicates the gear ratio necessary to transform a motor rotational speed of 5,000 rpm to the given wheel rpm. These numbers can be scaled in an obvious way for different velocities and motor rpms. (See the Quick Link giving various motor parameters.) Remember that the motor slows down with load, and the maximum power output of a dc motor occurs at about one-half of the maximum motor speed. Paradoxically, sometimes increasing the gear ratio can actually increase the velocity of the robot, since the motor(s) will then operate at a point of more power output to overcome frictional losses and thus rotate faster. You can download an Excel spreadsheet to do these calculations for different values.

Other factors of importance include the weight of the robot, friction in the gear box and drive train, and the number of motors used to drive the robot. So experimentation, guided by these calculations is necessary.