275/35R18 hoosiers on 18x8's
 

For anybody searching to see if they fit here are some pics: http://www.idanim.com/main/autocross/g35c/mods.html. Haven't had a chance to autox on them yet. I'm gonna add some 5mm spacers up front because there's some light rubbing between the inside tire edge and upright. Also there's only 2mm of space between the front caliper and my particular wheel.

275 is much too wide a treadwidth for an 8 inch wide rim. You will have some serious bowing of the tread surface, and may pop a seam under hard cornering. You should be running a 9.5 inch rim for that wide a tire.

I agree wholeheartedly with Ed. You should put those on at least 9". 9.5 would be optimal.

With that said, there have been many 350z owners using 275's on their stock ray's track rear wheels that are 8.5" and they're claiming no issues. I still wouldn't put them on 8.5" either.

The 275 hoosier is a cantilevered tire and is run frequently enough on 8" rims of rx8s and 350zs that I'm not too worried about tire failure. I'm limited by the rules on running a stock size wheel. The 2006 BS 2nd place 350Z was running 275 kumhos on a 7.5 rim I believe. I agree it's not optimal and if I could I'd run a wider wheel. I'm probably going to try a 245 kumho on 8" rim at some point too.

Here's one reference: http://sccaforums.com/forums/thread/171429.aspx. A search on the site should yield some more for those still concerned.

Correct and congrat's for knowing the score before you posted, though I would have posted on SCCAforums vs here, I do understand that their's not much traffic their.

However, as someone that just came off of 265's on 8" wide rims that granted were not R-comps, I won't be going back to such a tire and rim mismatch ever again. Two reason's, in the size I ran, 265-40-18 the narrow rim pulled the inside and outside edges of the tire down and reduced the total tread contact patch. Also, even though BfGoodrich KD tires have very stiff sidewalls, it didn't prevent a large amount of flex that would not have been present had I been running wider rims. We all know that sidewall flex isn't a positive thing handling wise, where it cost me the most that I could sense was in less them smooth corners where I would bounce on the sidewall flex and cause a reduction in avaliable grip. Never again for me.

Alot of the fast guys in STX seem to run 235's on 8" rims even though the rules allow for 245's. For those not familiar with the class it requires street tires. This is possibly for the sidewall flex issues mentioned above and also for gearing and tire availability reasons. I think the STS guys are running 215's on 7.5" rims for similar reasons. Seems to be a trend to go narrow with street tires, but I hear differing opinions on going wide with r-comps. Just thought I'd try it for myself.

Can't hurt to try, I guess, especially since you're only autocrossing with them. I would definitely think twice about taking them out on the track where you're going to be seeing triple digit speeds. Not worth the risk out there IMO.

I agree with Ed and Al; the R-comps are a lot stickier than the street rubber for which most of the size recs are made.

I'm not sure what you're trying to say here. Care to clarify?

ChicagoX can chime in for himself, but I think what he means is that the grippier R compound is going to be more likely to peel itself off the rim under aggressive cornering, long after the less grippy street compound would have let go and started sliding.
On a track, whether it be autox or road race, it is indisputable that it is a place where you will be pushing your tires to the edge of adhesion. the consequences of rolling out of the rim on an autoX course is unpleasant and embarassing, but not likely to cause too much damage and no personal injury. But blasting through Riverside at 110mph on Buttonwillow, or the Roval at Cal Speedway at 145mph and having it separate could get you killed very quickly. I just would not cram more tire on the rim than the manufacturer recommends for a road race scenario. I have no opinion on autoX.

T1 guys run 315's on 10" rims. This is outside of the recommended 11-12" range for the tire size.

http://sccaforums.com/forums/thread/87179.aspx
https://www.hoosiertire.com/rrtire.htm

If it was actually a issue, it would be banned in SCCA sanctioned event's, people do it, tire's don't come off and they're getting podium finishes. It didn't work for me, but it works for a lot of people have have reason to go that route.

Sad to say this guy is now talking and recommending these oversized tires to other people and its just not safe.

http://www.my350z.com/forum/showthread.php?t=290501

My goodness. I'm not recommending anything only stating that this is what the fast guys are running. Each individual needs to determine if the competitive benefits outweigh the risks. There exists a precedent supporting that this may be safe as stated above.