Ok well I'm going to order my new wheels tomorrow and the 18 inch ones are only $30 more than the 17's. Can I fit the 18's and what size tire will be roughly equivalent to a 255/40/17? I have lowered the car about 1.5 inches with Eibach springs if that factors in at all.
Also will the 18's offer any performance advantage in the handling department?

 

Actually what brand did you get you have to make sure the camber is right or they won't fit. Actually they will hurt your performance those mess with your rear I hear. Your camber wil be off and need to be adjusted also. Good LUCK with them. I hope it turns out well for you.

 

First off, 18's are too big for a 3rd gen. They just weren't designed to fit and offer handling/performance. If this car is for show then 18's would look great on it, the point is that handling will be severely hampered because the sidewall of the tires won't flex at all. This means they will just slide/spin instead of gripping. You will need to make sure that the camber angle isn't too severe also. That will hamper performance too, just because F1 cars are like that doesn't mean a street car's suspension is designed for it. Basically look for reduced performance. Its up to you. 17's or 18's.

 

I would get 275/40/18 in the rear and 245/45/17 in the front! or 245.40.18 in the front!

 

in figuring it out for my car, the 275/40-17 on a 17 x 9.5 wheel with 5.5" backspacing works. many mags have done it. now with an 18" use 285/35-18 tire on a 18 x 9.5 wheel with 5.5" backspacing. the two tires are both 25.7" tall. yes you can have some handling loss because of very little sidewall.

but use a 18 x 8 with 4.5" backspacing and 245/40-18. it is also 25.7" tall. the 17" tires will have more sidewall for more flex.

 

Ok I'm going to go with 17's. The wheels are only 17 x 8, but they only cost $499