This is my 235/45r/18. I can't imagine that your tires aren't rubbing on that spring perch.

 

First, as always, define your terms.

Google around and see what “coilovers” are offered for HHR/Cobalt. The OEM struts are already “coilovers,“ i.e., the spring and the dampener are coaxial. And at least some of the available sets don’t provide coilovers in the rear at all. They provide a different shock and a different spring. So in the vernacular, there are “lowering springs” which just springs, and “coilovers” which are sets springs and dampeners, virtually all of which provide adjustable spring height, a.k.a. “ride height” in the front, and may provide the same in the rear. Some also provide dampening adjustments.

The coilovers available, if they change the position of the struts spring seat, do so by… changing the position of the spring seat. But virtually all of them, equally importantly, make the diameter of the spring smaller. Presumably, these things are done specifically to provide more room for the tire.

And yeah there are negative effects too making changes to accommodate larger tires, some of them quite significant. But keep in mind that while some of these changes are made for the track, some or maybe most are motivated more for style than performance.

A few times, in other formums, I have suggested that the HHR SS suspension is pretty good as is, for street performance, anyway. It falls flatter than flat, could get a hostile response or ridicule.

 

I made some tweaks to the chart. All the same tires tho.