[JesseK]

In essence (be sure to keep units in check):

Robot Speed (feet/sec) = Wheel Circumference (feet/rotation) * Wheel Rotation Speed in (rotation/second)
= [Pi * Diameter] * [Motor Rotation Speed / Gear Ratio]

Assuming Diameter = 8" (0.67 feet) and Motor RPM for a CIM is ~= 5400RPM (90 rot / sec)
15fps = Pi * .67 ft * 90 rot/sec / GearRatio

or

Gear Ratio = Pi * .67 * 90 / 15
= 12.65:1 for a 15ft/sec bot with 8" wheels.

Feel free to put in the numbers for 4ft/sec.