If I go with 18's on my SS 2.4 how much on an affect would this have on the speedometer?

 

Next to none if you get the right size tires to match.

http://www.discounttire.com/dtcs/infoTireMath.dos

Check out this nifty calculator...if you go to 18s and use the same size tires as ss/sc, it will only throw you off by about 1%. Hardly anything to worry about.

In comparison, the people switching to the 225/40 tires on the ss/sc are off by about 2%.

 

I do not want less sidewill height than 45's

 

I'd say go with the stock tire size, but the 225/40 has a much better selection and generally run cheaper. I know I'll probably run them when the time comes, unless there are more 215/45s on the market. Just too expensive and not enough competition for my taste.

 

In the case of the SS/SC stock tires (215/45ZR18) and the SS stock tires (205/50ZR17), the sidewalls are 96mm and 102mm respectively. In all honesty a difference of 6mm is never going to be noticed by the eye, unless you sit the tires side by side off a car.

On the subject of a 225/40, even at that you are only off by 12mm or half inch for the US residents, I'd say at point you may start to notice the smaller sidewall, but it's nothing extreme. a 3.5inch sidewall is still a fairly large sidewall, and again only 6mm less the the stock SS/SC tires.

EDIT: I'm a bit peaved currently, because I just spent the past 15 minutes collecting data for the editing of this post, only to have my computer randomly reboot on me, and to lose everything. Anyway, back on topic I guess...

What I was trying to get at before was that in the worst case cenario you'll end up with a 6mm smaller sidewall then the stock SS/SC wheels, which to the eye will never be noticed, and isn't going to effect ride quality enough to worry about it to much. Going with a 225/40 setup will allow for the car to get a better grip on the road, giving you better control in most situations. Not to mention, if you compare the data I have provided bellow, the overall diameter of the 3 tire sizes, the smallest difference is at 0.4mm going from a 205/50 to a 225/40, and the largest difference being 13.9mm going from a 205/50 to a 215/45.

You will get much less speedo loss (0.1%) with a 225/40 then you would (2.2%) with a 215/45 in the case of the non supercharged SS models.

205/50ZR17
sidewall: 102.5mm
radius: 318.4mm
diameter: 636.8mm
circumf: 2000.6mm
revs/mi: 804

215/45ZR18
sidewall: 96.8mm
radius: 325.4mm
diameter: 650.7mm
circumf: 2044.2mm
revs/mi: 787

225/40ZR18
sidewall: 90mm
radius: 318.6mm
diameter: 637.2mm
circumf: 2001.8mm
revs/mi: 804