a 315 is the thread width, the tire height is dependent on the aspect ratio, usually 70 or 75 (the sidewall height is either 70 or 75% of the thread width). it also depends on your wheel size. so basically a 315 can be a 31 or a 37 or anything in between. so if your running a 22" wheel w/ an aspect ratio of 75% and a thread width of 315mm, the overall tire size would be 40.6. someone w/ more knowledge please correct me if im wrong?:dunno:315's are 35"+- FYI get that piece of crap K&N off before you loose your engine!!! Get a Tymar.