My new rims are 18 x 8
What exact size should I be looking for?! 225/40/18 or 235/45/18 or 235/40/18?! I am planning to do some autocrossing.

thx

 

i say 245/40 but why 18's if you want performance?

 

good question... there's been a lot of arguing in this forum recently about tire sizes and it's making me very paranoid. i have the same size wheels en route and am freaking out over this.

 

It's kinda funny, because I have same wheels as well 18x8

Here's what I found out by searching.
Your choices are
225/40/18
235/40/18
245/40/18
255/35-18

The stock wheels have a 25.32" dia. If you use
225/40-18 = 25.086" dia or about 1/4" smaller.
235/40-18 = 25.40" dia or about .08" larger
245/40-18 = 25.72" dia or about 3/8" larger

The smaller diameter will probably allow quicker accelation, but will cause a bit more revs per mile.
The larger diameters will slow the acceleration a bit more (the bigger the slower) and will give you less revs per mph, but will also show less speed than you are actually travelling.

If it were I doing the choosing, I'd go with 235/40-18 because it's the closest to the stock diameter and the same width as stock.

Alot of people tend to have 245/40/18 with 18x8 wheels

 

i realize the choices... the issue (for me) stems from weird claims of size increases and rubbing with my springs. i'll probably stick with 235/40/18...

 

255 40 18 without a doubt. I have 255 40 17s on my stock wheels and 275 35 18s on my 8.5" wheels, and no bulge at all. more tire=better.