I bought a used HHR with 30,000 miles on it that was owned by Avis in Florida. I let the renters break it in and pay the depreciation on it. It gets 23 mpg in the suburbs, and 36 mpg at 60 mph on the Interstate. It works perfectly. I am completely satisfied with it.
http://www.trfindley.com/pghhr.html

 

My 08 2.4 2LT Auto's DIC shows a steady 23 to 24 average. The best I've ever gotten was 28.73 on a road trip. Running mathematical is 24.68.

I live in Seattle and it is a hilly area... lots of ups and downs and I think that drags down my average.

Also, feather footing it helps with my HHR. Lead footing gets me 2MPG less probably...

I won't gripe.. my next road trip south (its flatter than East-West) I'm gonna try to break 30MPG.

I think I'll have to use Cruise control and keep it at 60 or 62. Doing 70 there is a noticeable drop.

Just some random thoughts.

 

I would love and pay for a mod on the HHR that had a split hatch like the picture on the bottom of your page of the 50's Chevy!