[wingnut60]

" If you have the older 7,000 lb axles, going to the 17.5 tires will not keep the axle from bending or going out of alignment. The new tires will wear out very quickly and unevenly. The 17.5 tires are also more expensive than the 16's so you will be losing even more money."

I respectfully disagree with this statement, unless the trailer has way more than 14k on the axles themselves. I, and several others, have made the changeover to the 17.5" wheels/tires and have not experienced abnormal tire wear. An 18K gross trailer with a 4200lb PW will have 13800 on the axles--close, but not over. I made the change and put over 25000 miles on the XTAs with no problems in tread wear or flats/blowouts.
This last spring, I put the 4k spring paks on in place of the 3.5k paks, and the axles were both: still arched, and in alignment.
Joe