235 40 18 fits the oem rim with no clearance issues.

 

But that size is .6 inches shorter than the stock 225/45/18's on the SS.
I would want it a little taller to have more sidewall. Been covered before that you can go up to about 26.3 inches in tire diameter.

 

dopefiend951 - If you are running 215/50's on a 9" wide rim you are way out of spec for that combination. You might want to read the details at this page: Tire Width for a Wheel/Rim Size Chart

Also, having your rims/tires extend past the outside of the fenders is illegal in most areas.

The forum here, cannot support such modifications at all. Therefore your post has been deleted.

 

Just did 18x8 +40 wheels and 255/40/18 tires with no clearance issues

 

I'm running 235 45 18 Michelins on my 2009 SS on factory rims.
they come close to the lower spring mount but they do not rub
they look really good on the car

 

For what it's worth I'm running 245-40-18 on my stock 18 SS wheels with no clearance problems.

 

I have a 2011 chevy HHR and I want to know if I am able to fit 205/65/16 tires on my rear wheels only? I don't want to put them on if they will not work.

 

You have to run the same size tires on the front and back. The ABS speed sensors will go bonkers if you run different sizes.

Fwiw. That 205/65/16 would be too tall to fit on the front. Also not sure why you would want to use a skinnier tire than the stock recommended size.