I have an autocross event coming up next weekend and I'm planning on taking my IS. I have been running wider 235/45R17 rear tires on the stock 7" wide wheels for several months now on the street with no problems whatsoever. I know that some people say this isn't really a "safe" size tire since its pushing the limits of proper mounting width. Like I said, the tires feel pretty good, but I haven't really pushed the car to its lateral limits too much on the street like I'm about to through the cones. I'm just wondering if it's ok to autocross these tires on stock rims and if anyone else has done it before with the same combo without any issues. I just don't want to get out there and by pushing my car hard around a corner and have one of the tires come flying off the wheel, especially if I drop the tire pressure.

 

You've already brought up a valid concern, the width of the tires to the rim. I have 205s on a 6.5 inch wide rim and they are bubbled out. You failed to mention the tire manufacturer. Some are known to be more square and others rounded, so it will be hard to see how the tire looks on your rims.

See if you could do a slalom in a huge empty parking lot. Just set up 3 cones and see how it feels. Start SLOW. Increase the speed with each pass and see how it feels. Check for sidewall roll. Thats all I can think of.

Your tires are not only wide, they are tall for the car. It's throwing off your speedo and odometer.