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I wanted share my experience with sizing tires for the 17" rims that come on the 850R. I have read a lot of posts on the board and searched the archives for sizing answers and found many contadictory opinions. I have some answers that some people may be looking for. I have now bent two of my Beautiful stock 7 spoke rims that came on the 850R and ruined 1 tire from pothole impact(and trust me I don't aim for potholes although I do drive aggressively). As you know these rims come stock with 205/45/17 Pirelli Pzeroes. These are great handling tires but provide a lousy ride and very little safety for the delicate 850R rims.
I have just fitted a full set of taller/wider 225/45/17 Goddyear Eagle F1 GS-D2 Ultra High performance tires to the stock rims(Very cool looking formula 1 derived rain tire tread). Purchased from tire rack and I must admit they were extremely helpful and a sheer pleasure to work with. AAnyway, they fit well and provide a significantly plusher ride then the stock tires as well additional Rim protection(taller sidewalls and larger circumference). They also provide more sidewall protection to help prevent marring of the rim lips from curbs and carwash rails). The car no longer CRASHES over small bumps/cracks/potholes. It now provides a pleasant/comfortable thump. However, here are some things to consider. I must admit that I have lost a bit of the razor handling of the lower profile Pirrelis. Also, the taller tires do change the final drive ratio a slight bit and the quickness of the car will suffer slightly but offer I little more top end. Also this change will throw off the speedometer, I still need to calculate the difference. Finally, and most important, the tires will slightly rub the inner fender well when you have the steering at full left or right lock and you initiate some body lean/roll. I have not found this to be a hinderance as it is pretty rare that I ever drive at full lock and if you back off just a slight bit it does not rub. As far as normal driving,hard cornering or bump compression is concerned the tires do not rub or hit anything.
A 215/45/17 will also work fine without any rubbing at all. A 205/50/17 may also work(somebody else will need to test this one). So far I am very pleased with the new tires. It feels like a different car, more refined and not as nerve racking to drive as I was previously always worried about bumps in the road and the damage they would do....Hope this helps. Now I just need need to upgrade the Motronic, exhaust and brakes and its smooth sailing. A great site to compare tire sizes is "www.powerdog.com/tiresize.cgi" This concludes my PHD dissertation in Tire anatomy.
Matt T.
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