Ok far from a math major guess should of listened to mom and listened in school.
When you measure the Diameter of the tire is it taken from rim to rim edge
or the outside edge of the foam or rubber tire. I'm thinking foam edge to foam edge across the center. Is this correct ? Thanks and no more stupid questions till the next one.

Take your calipers and span them the length of the tire from top to bottom and you should just barely be able to move the tire inside the calipers if done correctly..... I am sure someone at your local track can show you how to properly do this if your still unsure, I would always ask someone local, there are folks out there with all kinds of knowledge and willing to help out.........

the size of the foam is what you check.. as the foam wears you have to adjust the gearing.. the smaller the tire gets, the more you have to gear up..