After learning that the HHR SS had larger diameter tires than the Cobalt SS,
HHR 225 45-18
Cobalt 225 40-18
I was looking some things over on tirerack.com and notice somthing interesting.

For us SS T/C owners, we have few options for wheels because of our brembo calipers.
On tirerack.com you select 08/09 SS T/C model and your choices are tires and brake no wheels choices.

However when you select 2008 HHR SS you have many wheel choices. Why won't these wheels work on a Cobalt. I thought HHRs and Cobalt were on the same platform.

Is the HHR bigger? are the rotors, calipers and such different from the Cobalt?
I am pist that I learned that the HHR's gas tank is 3.2 gallon larger!! I need a HHR gas tank. lol

I was surprised to see that the HHR SS has tires that are .8" inches larger than the cobalt.
The HHR SS only gets 1 mpg worse than the cobalt and it has a lower 4.05 final drive ratio compared to Cobalts 3.82. I wonder how much of an increase Cobalt owners would see if they put HHR tires on. (It could look stupid) If it was only a .5 inch different I would do it.

 

well right now the hhr SS doesn't have the Brembos that why they wont work, im sure the Sotck HHR SS wheel will fit the cobalt TCs

 

Why haven't they been making the HHR SS with Brembos yet? This sugguests the brakes on the HHR SS are different from the cobalt ss tc.

 

HHR SS is not as performance-oriented as the Cobalt SS. Brakes are less beefy, and the thicker sidewall is for a little more comfort on the road. The larger tire diameter is offset by retaining the old Cobalt SS's 4.05 final drive.

 

supposedly the late 09 models you can get them as a performance package.

 

If you put 225/45 tires on the Cobalt instead of the stock size, your speedo will read approx. 3.5% slow.

i.e. a 60mph reading means you're really going ~62. As for effects on fuel economy, it will be minimal.

This is a great site to compare tire sizes.

http://www.miata.net/garage/tirecalcold.html

 

I used http://www.angelfire.com/fl/procrastination/rear.html to come up with these rpm numbers. The speeds are measured at 2500 rpm with a .70 overdrive.

HHR SS- Final drive 4.05 with 26" tall tires 68.20 MPH
Cobalt SS- Final drive 3.82 with 25.1" tall tires 69.81 MPH
Cobalt SS- Final drive 3.82 with 26" tall tires 72.31 MPH

I guess it would only lower your rpms buy ~100 at 70 mph. I would have thought it would be an easy way to pick up an extra 1 mpg. Interesting