If I got a rim at 17x8.5 and tires (Hoosier a6) of P225/45ZR17 which have a 8.8" tried width and was lowered by 2"-2.5" would I have problems with rub or clearence?
I ran Hoosier A6s in 225/45 x 17s for one year with stock spec (and stock) rims -- 17 x 7 -- with no problems -- I took a National Tour event in Texas two years ago with this wheel/tire combination. I ran Hoosier 225/40 x 17s on stock spec rims without any problems last year. Bottom line, you don't need to buy 8.5" wide rims if you're only going to run 225 rubber!
I presently have a set of Falken Azenis 615s in 245/45 x 17 on a set of 17 x 7.5s -- with a 40mm offset -- and everything clears just fine (it's close but it clears). The Azenis are 9.1" wide and are actually about a tenth of inch narrower than the Hoosier 225/40s -- they're also 1" larger in daimeter than stock -- 25.6" vs 24.6" -- and they clear with my Progress springs and work great as 'rain tires' -- they help reduce the torque load on the fronts when it's wet, so I'm not spinning the tires all the time.
I also have a set of 18 x 8.5s on which I have mounted Hoosier A6s in 255/35 x 18 (these have 10.3" of tread width and they're 0.2" larger in diameter than stock wheels/tires -- 24.8") and they just rub on the inside with a 35mm offset. They rub in front on both sides, they rub about a 1/4" spot on the upper ridge on the struts with 1.5 degrees of negative camber (I think I really need an extra 10mm of spacing up front and I think I could reduce the camber and not rub at all) and they rub on the inside of the right rear (over the liner where it covers the fuel filler). So I'm waiting on delivery of four 15mm offset Eibach spacers to run these tires (I've been waiting since early February for these from Gravana and they're apparently 'back-ordered' and taking a very long time to get delivered -- it's coming up on two months waiting)!
I have used my trusty Dremel tool to grind away the sharp edges of the front wheel wells to get about a 1/4" of extra clearance (it leaves the lowered edge of the front wheel wells flat, no sharp edge) since I run Progress springs and the tires stick about 1.5" outside the wheel wells. I've purchased a set of NASCAR-style spring rubbers in 'hard' for the front springs and in 'medium' for the rear springs. With the spring rubbers in place, I don't rub the wheel wells at either end and it actually seems to help with the spring setup -- I think I'm going to become a fan of 'spring rubbers'. Easy to put in on race day, easy to take out afterwards.
Finally, I have a set of slightly used Kumho V710s in 285/30 x 18 (again, 0.2" larger than stock wheels/tires -- but 11.3" wide) that I plan to put on the 18 x 8.5" wheels as soon as I get the spacers to I can see if I can run them too. Where the Hoosier 255/35 is 10.8" at the widest, the Kumho if 11.4" at the widest -- a net of 0.3" of additional width on the inside -- about 10-12mm -- so I'll have to see how they fit. Further, the Kumos want a minimum 9"-10.5" wide rim, so squeezing them onto an 8.5" rim might make them a bit narrower than spec -- I'll see. I've seen guys cram some really wide racing rubber on some seriously under-width rims and everything works out just fine.
BFG also has some tires that are in the same width range but they're harder than either the Hoosier A6s (the A6s are definitely the softest/stickiest rubber) or the Kumhos (not quite as sticky as the Hoosiers, but better wear). I want to run the Kumhos to save the Hoosiers for the two National Tour events I'm registered to run and for the Nationals in September. I can't afford to buy more Hoosiers @$300 each.
I'm lucky, I happen to live and drive in an area with lots of SCCA racers so I got the Falkens for $225 from a Mustang Cobra ST racer and I got the Kumhos for $100 from a fellow DSP BMW racer.