I heard they're based on a Cobalt platform. I saw a stock Cobalt SS run 14.50's up here in the altitude (4500 ft). I'd Imagine an HHR isn't too much heavier? If it could pull 15.0's that's fairly respectable, my wagon with 2.41 factory gears and a torquey 400 has only run 15.70's at altitude. The HHR probably gets twice the mileage and is probably faster!
Is yours a panel or a regular HHR?