Let's be clear. The axle and suspension are made by Dexter Axle. At their web site - specifically on the second page of the 3500-pound Torflex axle description at
http://dexteraxle.com/i/u/1080235/f/...0-3500_lbs.pdf
The heading says "Allow 3-inch bump clearance from full load."
I take this to mean that if the suspension compresses (rotates) all the way to the stop, the tire will rise 3 inches from its rest position, where "rest" is defined as a full 3500-pound load on the axle.
Therefore, to be perfectly safe, your TM, sitting still, should have 3 inches of clearance from the top of the tire to the underside of the wheelwell liner.
I made a conscious decision to go ahead, even though I had only a 2 1/2 inch clearance. Although my TM may be fully loaded at times, my expectation is that I will never hit a curb or a big bump at a high enough speed to fully compress the suspension. If I do hit a bump this hard, then I accept the consequences of my decision.
The consequences are unhandy, but not severe. The wheel well liner is a plastic molded piece, arched in shape, mounted above the tire. It is screwed to the TM floor inside the TM, covering the rectangular opening in the floor. If the wheel rises high enough to hit this liner, most likely it will simply push the liner up and pop the screws out of the floor. It might destroy the liner, but since it has a slick surface, I doubt it, and a new liner can be ordered from TM.
To avoid this possibility, I considered increasing the height of the liner. This would involve opening the door of the kitchen cabinet where the liner is located, unscrewing it from the floor, screwing pieces of 1x1 wood to the floor around the opening, and then remounting the liner on this "lip". In the end, I decided not to bother with this. But if the clearance on your TM is close enough to make you nervous, it should be a quick and easy fix.
Remember, of course, that this discussion applies to the clearance above the tire, but you still must consider the clearance ahead of the tire, aft of the tire, and on the inner side of the tire. This was all discussed in one of the early threads discussing 14"-to-15" conversion.
Bottom line - I simply mounted new 15-inch Goodyear Marathons on new wheels, and put them on in place of the old 14-inch Marathons. No other changes or mods. Thousands of miles later, no problems whatever. Thanks, Bobby!
Bill