I think F 245/40, R 275/35 is as wide as most people dare to go, and even then it's iffy, and fender rolling is usually required, at least for 18's and 19's. Probably more flex with 18's.
Although stock xi suspension might allow for it without any issues. Not sure though. What most people consider "ideal" is 235/40/18-19, 265/35/18-19 with spacers to adjust for desired aggression/stance. Depends a lot on the wheels you'll be running them on too.
Last edited by Kalidian; 12-27-2012 at 08:11 PM.