If you carefully look at either the rear steer wheel, or the front drive wheel, you'll see where the rubber adheres to the steel.. The inside of the steel/rubber adhesion is the actual press on tire.
Now if you again look carefully, you'll see the hub (or wheel), that peice should be all steel, therefore the tire is fitted to the hub w/ steel on steel.
Now if you measure the outside diameter of the hub, (which is the inside diameter of the tire) where the 2 steel(s) meet, that would be 1 measurement of the 3 needed.
The next measurement would be the width of the tire, from 1 side of steel, to the other side of steel. That would be your 2nd measurement.
The 3rd measurement, or tire OD, which is rubber to rubber height, can be determined by the 1st 2 measurements in the book.
So if the drive tire has a width of 7" and an ID of 12 1/8" the tire OD is automatically 18" (unless you have an uncomman size, unlikely). Therefore the tire size is 18 x 7 x 12 1/8.
This only works for cushion tires, not pneumatic or solid pneumatic tires. Hope this helps