OK, so I finally found the rims that are going to be going on my car, and they're 18x8.5 with a 38(mm I assume) offset. I plan on using a 255/45/18 tire all the way around. What I'd like to know is would that be enough offset for clearance for all 4 tires? I seem to remember reading here the stock rims were like 42 offset. What else should I look for? Backspacing is different, right? And we need like 6" backspace, again, seem to remember reading that here? I know people get tired of answering same questions, but what the hell else you going to do at work?

 

Stock 18x8.5 has a 50mm offset, the 17x8 is 42(ish).

For all my sizing quandries, I use the fact that the MustangTuning.com and Roush 18x10s use a +45mm offset which provides 7.27" of backspace (width + 1" / 2 * 25.4 + offset / 25.4), and 3.73" of frontspace (width + 1" / 2 * 25.4 - offset / 25.4). So, presumably any wheel nestled within those dimensions will work fine, which your 18x8.5 +38 wheel is.

The only real unkown is the wheel bearing. Usually it is advised to stay as close to the stock offset as possible to prevent excess load on the bearings that they weren't meant to handle.

 

It is tricky to compare offset and backspacing between different WIDTH wheels.

In other words, if you install a wider (or narrower) wheel, then the offset/backspacing measurements will be different than stock.

I would buy wheels that are specifically made for your car. If you are unsure, take a tape measure to your car and verify that there is enough clearance.

 

18x8.5, then anything with 42mm or less offset will work fine. I have 18x9 with 24mm offset. The less positive the offset the more the wheel will stick out.