Ron

Will overall wheel size change 17" from 18 inch wheels require either a alignment or a ride height adjustment, I am using PSS10

I was thinking that with the right tires they would maintain same height

Mark in Baltimore

No adjustment needed.

Foxman

+ 993. I do it every winter on the C4. They're the same circumference as you surmised. Good luck with the wheel search Ron.

Ron

I will definitely need luck with the wheel search. I want E 88 it seems that BBS is having trouble supplying parts to make wheels. If I had to guess I would say there some business going on behind the scene and somebody or some entity is trying to buy them out or acquire their assets.

RonCT

I've thought about this many times before. The suspension people I've worked with in the past (ie: Musante, Jacobs, AAOC, etc.) have all said "have the wheels / tires you intend to drive with on the car when we align", so I suspect there's a reason. Corner Balancing? So maybe 17 vs 18 doesn't matter, but the alignment can be off if you "change" wheels and / or tires?

race911

How? You're measuring off of hard surfaces--metal wheels bolted to metal hubs bolted to metal suspension bolted to metal frame. I did this extensively in the '80s to prove certain things to myself when I got my Hunter rack/alignment system.

(Yeah, throw on an alternate set of wheels that are grossly underinflated, or have a laughably bad wear pattern and you'll slightly change the parameters.)

RonCT

As I suggested, it has nothing to do with 17 vs. 18, rather I think it has to do with the particular weight combination of what you have that can influence something like a corner balance. For example, say you have 50 / 60 lbs total front and rear per wheel / tire. And you change that for 45 / 50 and save 5 lbs up front each and 10 lbs rear. I suppose that would throw things off ever so slightly.

Thus, I've been asked to have the wheels and tires I intend to run on the car for when they do suspension work / alignment. And that's the only thing I can think of...

600RR

They're the same circumference as you surmised. Clarification please:
OEM equipment:
205-50-17 = wheel diameter of 25.08"
255-40-17 = wheel diameter of 25.04"

AND

225-40-18 = wheel diameter of 26.08"
265-35-18 = wheel diameter of 25.30"

Foxman

Those wheels are compatible. I don't think you have the right figure for the 225-40-18. The circumference is actually 25.1" almost identical to the 205-50-17. The circumference does not have to be exact, but should not vary more than 3 to 5%. See the attached tire calculator link.
http://www.wheelcalc.com/#.

Ed Hughes

The questions were alignment and ride height needing adjustment. While I have my car fairly well dialed in on CB, l'll bet most aren't. Then, throw something in the trunk or a passenger in the RH seat and all bets are off.

race911

Or, on the 99%+ of these out there which do not have adjustable stabilizer bar drop links. Amazing how much preload is introduced with the stock components.