Got a 09 Ralliart lowered on H&R Springs and have a set of Brand New 235/40/18's.
My question is, what wheel width and offset do I need to avoid rolling the fenders?
I've read the fitment guide over and over and there's a couple sizes that say they'll fit perfect with or without spacers but don't really mention if they'll work after the car has been lowered.
I have a nice set of 18x7's but am told the 235's are not a good fit.

 

This is the issue. If you want to really use the entire width of that tire, you need a proper width rim to do so. I actually have 235s on my car (went down from 245s for just a tad more clearance). But honestly, a LOT of it depends on tire geometry. Some tires have significantly different sidewall geometry, even in the same exact size.

But to answer your question, early on, I had X wheels (18x8.5, et38) with 225 tires, and they scrubbed the rear BAD. I had the fenders rolled instead of getting higher offset wheels, because I really like the way the car looked. When I went up to the 19's, I stuck with the exact same width and offset rim, but went up to a 245 tire, and actually had slightly more clearance than the 225 that were on the 18's. Go figure. So, what I would suggest is this...

Ask yourself if you are ever going to upgrade the brakes.

YES: You really should go with a wider wheel, lower offset to be able to clear a BBK. Evo X size is a great target, if you don't want to go too aggressive, and all that will require is a slight roll in the rear (even though some have claimed they went up to the X wheels with no rub, but that is very hard for me to believe). But my X size wheels barely clear my 6 pot Brembo kit. Some clear by a mile, and some are very close like mine. It really depends on spoke geometry, and if the wheel has a lip, or not.

NO: You should target a wheel in the 8" wide range, with an offset between 42-48. This is just a ballpark, because as mentioned, all tires are different.

My $0.02. Best of luck

 

Thanks for your input Drew, that helps alot. I was leaning towards a 8" wide wheel but the offset numbers screw me up. I don't plan on upgrading the brakes anytime soon and the tires were free and are General G-MAX.
Just trying to avoid rolling the fenders. I'm prob not capable doing it myself and don't think anyone in my area can do it either

 

Anytime. But if a set of cheap X wheels, or some other with a lower offset pop up for a good deal, Trackwerks looks like they could do your rear quarters (fronts wouldn't need it) without issue.