Some German car with an M on it (99 M3 Coupe)

[SlimKlim]

From the SCCA Rule Book:

Any type wheel may be used provided it complies with the following: it
is the same width and diameter as standard and as installed (including
wheel spacers if applicable) it does not have an offset more than ±¼”
(±6.35mm) from a standard wheel for the car. The resultant change in
track dimensions is allowed. Tire pressure monitoring sensors may be
removed.
Wheel spacers are permitted, provided the resultant combination complies
with the offset requirements of this section. Wheel studs, lug nuts,
valve stems (including pressure-relief types), and/or bolt length may be
changed.

Assuming you're trying to keep it in stock class. If you're going to put suspension on it it bumps you to STX or STU I think (its been a couple years since I've autox'd mine, the rules may have changed) and you can run up to a 245 square.

Unless you've got mad money to spend on some fancy forged 18s, I think you'd be much better off going with a lighter 17" wheel. I used to run 17x9 MT1-Rs that weighed around 17lbs each, and then when I started dailying the car to work after graduation, I went for some Icy Hot Stunna-status 18x8.5 cast CSL reps that are the approximate weight of a dying star. The difference in every aspect of the car's performance was astounding, slower off the line, slower to corner, heavier to brake, etc.

They do look damn good, and I'm still running them with a 245 cause my car is certainly no race car, so it's just some food for thought. I think the 18" reps are pretty close in weight to some of the stock wheels so the difference might not be that noticeable coming from stock. If you go for 18s, get some Apex EC-7s. They look sort of like a CSL but are cheaper than the real thing, a bit lighter than the knockoffs and have good concavity to them. I have a friend that works there so I might be able to get you better pricing.