Sorry, I forgot to finish my post. I don't think you can "buy tires that fit the rims" in a 4x4 tractor because the height of the tires makes a huge difference in the ratio. So you do have to buy a complete set of rims and tires matched for that particular tractor, no? Or, of course, you can replace your original tires with tires of the exact same size (height and rim), though you might be able to play with the width, you can probably also cause problems with clearances if you go wider and with overall width (and thus balance) of the machine if you go narrower. I have read the galaxy tires can hit FEL's on some tractors.
To take a simple example, if you had 18 inch rims and six inch high tires on the rear, that would be a total of 30 inches. If the same tractor had 12 inch rims and 4 inch high tires on the front that would make the ratio of the circumferences 2/3. If you replaced the tires with other heights, either in the front or back, it would probably be impossible to find tires for the other two wheels to duplicate that ratio. If you went out and bought a set of rims and tires of another type for that tractor, and measured them, the ratio would have to be pretty darn close to that 2/3 to be safe, and I'm sure they will be. This is a pretty simplistic analysis too because the actual diameter would be dependent upon how much the tire flexed or flattened at the bottom. Turf tires are designed to flatten out quite a bit at the bottom and this reduces the diameter, and changes the ratio as well. So, even if armed with the ratios involved, putting any combination that is not specifically designed for that tractor (or more accurately, that the tractor is specifically designed for) would probably take an engineering degree to do properly.
Or not.. I don't know..