For what use? road course or autocross?
For autocross, no worries, they'll work fine. For road course, I'd be inclined to go with a 295/30-18, for better turn in and contact patch shape. Note: I've never run that exact combo, but have autocrossed the 315 on a 18X10.5" rim with good results and have also run the 295/30-18 on an 8.5" rim on course and not been really happy with the feel...so, I'm giving kind of a educated guess, based on my experience and preferences.
Last edited by tigerdrvr; 12-30-2010 at 06:59 PM.