Premium works great in mine

 

I had a 2.4 in the 2006 HHR and have a 2.2 in the 2010 HHR.
Both had/have the 4-speed auto transmissions.

Not sure what they did besides adding VVT to the 2.2, but the 2010 2.2 seems to have more get up and go than the 2006 2.4.

 

No problems with mine. Runs strong get off the line and even better above 3 grand. I Drive works well and hits 110 all day long. ( Don't do this at home)

 

possible difference in gearing ?

 

Depends on what you are looking for. I have a 2007, 2.2 with a five speed. It has enough power to keep me happy, but then I'm not racing it, or doing quarter mile timings with it. My feeling is the five speed is really an advantage with the 2.2.