I recently completed a trip to Missouri and back from Southern California in my ’07 HHR. I had previously arranged to buy a small 5’ X 8’ utility trailer near Jefferson City to haul a Honda ATC back home with us.
Driving there was no problem, the car ran great while getting 28-29 MPG cruising at 75 MPH with the A/C on. The pendulum quickly swung the other way once the trailer was hooked up and we headed west! The HHR soon reminded me of the ’73 Vega I used to drive, absolutely no power. The transmission rarely would shift into high without feathering the gas pedal, and even then 65-70 MPH was top speed while mustering 20 MPG at best. Any head wind or side wind just made things worse. As a result, I would not recommend pulling a trailer long distance with an HHR unless you can get a low-profile version. The upright ramp, as seen in the picture, provided enough wind resistance to resemble a small parachute. The weight of the trailer and cargo was 700 lbs at most, so that was really not a factor.
Below is a photo taken in Arizona at Petrified Forest National Park.

 

verticle speed brake.....ok for local runs..would have pulled it off for a long run...I wouldnt pull that with any vehicle on a long run.

 

Vertical speed brake is a great definition for the ramp. I would have folded the ramp forward and put the ATC on top of it, but I had some doors laying flat that would have been damaged. Looking back I would have loaded it differently somehow. I was surprised at just how much reserve power the HHR doesn't have.

 

You didn't say what motor you have. I have an 06 w/ the 2.4 and pulled 1000lbs total weight 1800 miles w/out the "vertical wind brake" . Had plenty of power and got about 23 mpg.

 

My car has the 2.4. I agree that 23 MPG is attainable without the vertical air brake. On flat road with no wind, which is rare between Missouri and California, the drive wasn't too bad and the cruise would actually maintain top gear.
I should also make mention that whenever we stopped for gas I would let the car idle with the A/C on (for the fan) until the coolant temp dropped below 200.