This question comes up a lot from people who are newer to bicycle building.
FIrst off--the Sheldon Brown tire sizing page is probably the #1 place to start:
Tire Sizing Systems
I'm in need of tire guidance because there doesn't seem to be any rhyme or reason to the tire size numbers from one company to another. I have 3 sets of 2.25 x 26 tires and they are all drastically different in size from one another.
That's because those numbers are only approximations. 2.25" is supposed to be
about how wide it is and 26" is supposed to be
about how tall it is when inflated on a rim.
The width that any tire ends up depends partly on the width of the rim that it is mounted on, and there's no standard width for bicycle rims.
Assuming that you have a modern MTB or cruiser bicycle***, then the '26' part of the sizing indicates that the tire is a I.S.O. 559 size, because the tire's bead seat diameter is 559mm. The I.S.O. size designation of "559" covers tires from as narrow as 1" to as wide as 2.125". You can easily find MTB tires that are wider than 2.125", but that doesn't matter, they are still considered to be I.S.O. 559 size, because they still use the same bead seat diameter.
The I.S.O. tire sizing system was invented because measuring the tires and rims by their bead seat diameters is the only accurate way to do it. Previous systems usually tried to designate tires by their inflated height and widths, but rims come in different widths, and the width of any tire is partly dependent upon the rim it is mounted on. The I.S.O. sizing system makes no attempt to convey the overall inflated size of a tire at all, so the tire manufacturers and retailers are still left estimating that.
What's the biggest tire you can get in a Schwinn panther style frame? I originally wanted to mount some fat franks on it but I don't think the frame will permit it. ...
There's really not much of any way to predict, but try them and see. Every bicycle frame that was made for "26-inch" 559 wheels and tires will not fit every 26-inch tire there is. Rims come in widths ranging from less than one inch to more than two inches, and some MTB tires are rated as 2.5 inches wide. Due to the materials used in their construction, bicycle tires that are wider are usually taller as well. Even if a frame has enough width for that 2.5" MTB tire, the frame may still not have enough room for a tire that tall.
MOST cruiser and MTB bicycles I have ever seen can fit a 2.125" tire easily. Wider/larger tires than that can start running into problems with chains and fenders rubbing, and maybe even frames rubbing.
***(If you are using a vintage bicycle and it has "26-inch" tires, then you first need to identify EXACTLY what rim and tire sizes it has. There are at least three different common "26-inch" sizes of bicycle tires used in the last 80 or so years, plus a couple others that appear so close they might fit, but won't. All these sizes have different bead seat diameters, and are not cross-compatible. There is info on the Sheldon page about them)
Good luck!
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