Overall, you've got the perspective down for the most part. Just minor details like making your ovals true ovals and your straight lines straight lines would help. So if you tuned up those drawing basics it would polish up the design. As for this design itself, I like the retro-style fenders and round almost porshe-like headlights, but even if it is a retro design (which the HHR is) it should still incoorperate some of the current Chevy design cues. (Example, either a 3 section lower front fascia, of a grill with a "bar" running though it) You know what I'm talking about. =)
So overall, I don't mean to sound like a know-it-all, because everyone knows I'm not the greatest artist on earth, but the biggest hint that anyone can give you is keep drawing and studying different cars designs, because the old saying rings true, practice makes perfect. Keep up the good work!