What are the biggest absolute outside diameter we can get away with on our 2IS? I'm asking because I want the most sidewall I can get out of these oem wheels. Our factory OD is 25" Anyone running anything significantly bigger? I'm currently .4 inches bigger in OD and the local tire places are saying no more in terms of OD, siting I'm as big as I can go.

 

I'm assuming you have 18" stock wheels, since you didn't state. If that's true, stock size is 225/40/18 front and 255/40/18 in back. And, again, if that's the case then your current OD is 25.1" in front and 26" in back.

Lou

 

26-inch, lifted with rubberbands... Dunk!!!!

 

I have stock 17" and I know the factory outside diameters, I needed to know the biggest I could possibly go without rubbing any wheel wells or fender lips.

 

i ran with 225/3519s and 265/30/19s with slight rubbing issues at full lock only. it could be due to my ride height as well (not aggressive, lowered on H&R springs).
going oversized, you have to consider the lock-to-lock clearance in addition to fender-to-tire clearance.

if you are not lowered you can get away with a taller tire but not too much more with the lock-to-lock clearance to consider.

 

Back to your original question. Your wheels are 17" X 7.5". And as you say, your current OD is 25" with the stock 225/45/17 tires. As I mentioned above the 18" ISX50 wheels with stock tires have ODs of 25.1" and 26" front to rear. And, the ISF has 19" wheels/tires that have an OD of 26" front to back. So one can assume that coming close to 26" would work for you. However, it might throw off the electronics and will certainly throw of the speedometer to a small extent, and will have a negative effect on your acceleration. With that being said, a 225/50/17 will give you an OD of 25.9" and will give you what you are looking for.

Lou