First, look at the motor data sheet FIRST provides:
http://www.usfirst.org/sites/default...nformation.pdf
Then figure out, for your design, what the final speed you want to go... the final ratio you want is the multiplier that makes the free speed about the same as your desired speed (but please note... free speed <> speed under load. If you want it to be that fast under load, give your self a buffer and gear it for a faster free speed than you really want it to go).
Also for reference, 217-2000s are usually referred to by their name, CIM motor. The same for the mini-CIM (217‐3371) and the BAG (217‐3351) motors.