Use this tire comparison calculator, it also gives you the MPH differences.

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

 

Not sure about the 17's tire sizes, but at the dealer I bought my SS, I saw a HHR-LT with custom rims, and 235-40-18 tires. I checked and there was plentry of clearance.
So 235-40-18's will work on my SS when it's time for tires. With my Trifecia tune in, it won't be long. "Moon"

 

they were the stock firestones the tread looked like new, had a tire shop compress the sidewall many cracks the tread would have come off the tires were mfg. in 2006, firestone had this problum with fords some years back, went with Falken have had them before and liked them, i have never had a compression test on tire sidewalls, something to think with Firestones.

 

Comparing the 215/50/17 to -
225/60/17 is about .375" wider, 2.14" larger diameter, speedo 4 mph higher at 50 mph
235/60/17 is about .75" wider, 2.6" larger diameter, speedo 5 mph higher at 50 mph

Some dealers will not sell/install sizes outside those recommended by the mfg.

The wider the tire the less pounds per sq. inch per tire, may be of concern in heavy rain.

Here are some sites if you wish to compare the differences - height, width, revs etc..

http://www.csgnetwork.com/speedocalibcalc.html

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

http://www.rims-n-tires.com/rt_specs.jsp

 

Sounds like I either need to stick with the size that I have now or just go up to 225's but no more than that correct?

Blue

 

Blue-

That's what I'm gonna bump to is 225/50/17, little more.

The stock rims are only 6.5" wide.. So any bigger is gonna be too big for the rims imo..

 

235/40 18 is almost a 1/2" shorter then the 225/45 18. I was considering the 235/40, but wanted the slightly taller tire. I have about 1/3"-1/2" from the top of the tire to the bottom of the strut plate.

I don't know how they would work with stock rims, but some have done 245/40 on aftermarket and had clearance. That will take you to about a 25.7" tall tire. The 235/40 is about 25.40" tall.

 

I've pulled multiple calc.s and they all tell me that the 235/40/18 (and I don't mean that it has to be right, it's just a consensus) will set me about .2 mph faster when my gauge reads sixty, but is that because I had the 16's

 

No; thats because the overall diameter of the the tire larger.