Now for some numbers ---
Our stock tire is 25 inches tall. Our stock tire is 5.2 inches wide. We have 0.7" of clearance on each side of the tire. On the right we hit the lug that holds on the right hand passenger peg (this could get trimmed if needed). On the left we hit the belt guard (which could be relieved if needed). We have clearance to the U of the swingarm to the tune of .880" which cannot be moved.
The fattest tire we could theoretically swing without taking out the grinder would be 6.5 inches wide. Ideally, we want to stay at 25" in diameter or get even shorter as we get wider (wishing to keep some clearance to the top of the fender during bumps, etc. -- as you get wider you take out the radial clearance as a wider straighter flat gets inserted to the same distance into the same round air clearance.
If we were willing to grind, we could go to 7" wide (but we could count on having other complications at that width such as needing a fatter fender).
Our rim is 3.63" wide. VW tires were intended for a wider rim, so you can count on some slight bending of the flat part of the tread (and having one hell of a wrassel hassel putting the tire on the rim).
Look here and here for some data on current metric tire physical attributes.
WARNING: Many tires are shorter-thinner-and some are WIDER than the general specs listed here. Goodyear tires are generally WIDER than what is shown here, the cheap imports seem to run a bit skinnier as a rule of thumb. YOU NEED TO GET SOMEBODY TO ACTUALLY MEASURE THE TIRE, then you need to remember it grows a bit with air pressure.
http://www.angelfire.com/on/geebjen/tires.htmlhttp://clubs.hemmings.com/clubsites/chevylist/tech/tire_size_conversion.htmSo, not only are the VW tires listed in posts above possible darkside choices, but some new small 165 and 175 wide current 65 series metric tires could work as well.
Specifically, the new style Minicooper tire, the 175/65R15 just about totally fills out the maximum space between your swingarm rails -- but your real-world actual tire does not line up like that (rats, our stock tire is offset some) and there is a mounting boss on the right side of the swingarm that sticks out in the way making this a no fit.
But if it was easy, heck, anybody could do it.
Once you pick your actual tire out, get in touch with the seller and get him to actually measure the width and height as every tire brand is slightly different than the "standards" say they should be.
That's Darkside talk for "it .... could .... WORK"
(if you are careful picking the out the actual tire)
<a big grin & a crash of lightning and thunder>
Oldfeller