You just need to be concerned with the overall tire diameter. Whether you have a 14 or 15 inch rim, doesn't effect how many RPMs you will rev at a given speed if the overall diameter of the two tires is the same. The diameter of the rim will onlly affect your handling and ride comfort. Now, the width of the rim could effect the RPMS slightly because the tire side walls are either stretched or pinched if you have a wide or narrow rim.
On the specs for tires, there is usually a note that will say the given diameter and footprint is based on a # inch wide rim, where # is usally the recommended rim width. If there is not a note, you can assume that if a tire says it will fit a 8 to 10 wide rim, the specs are given based on a 9 inch.
I didn't mean to step on Psycho. He beat me to the explaination.
