Stage 1 GM performance upgrade. It might increase your mileage a tad, but it requires premium fuel. The SS without the upgrade can run on regular, but loses a little power.

My mileage is right around 30 MPG on the highway. In mixed driving to work etc is 23-26 depending on the time of year and how hard I am on the car. Mileage is the same or better than my old 06 2.4(both were automatics).

 

I replaced my 2005 Vibe with a 2010 HHR SS after the Vibe got totaled. Gas mileage is about 30 on the highway if you can keep your foot out of the boost.

I don't regret it at all. It's a blast to drive!

 

Stage 1 automatic shows 24mpg instant at 100+ mph.
That's not too bad!

 

Must be nice to have roads open enough to do this! There's too many people going slow in the left lane around here.