Not bad for a guy who hasn't chopped in a year haha
I noticed a few things after looking at the high res for a bit.
-First thing I struck me was that the background was a bit darker then the car itself, so it didn't blend properly. So I just added a bit of brightness to the floor.
-I can still see the left rear tyre of the car that was originally in the background
-The fins on the left side of the diffuser is off angled, so draw a few lines and move it to the other side to check the perspective.
And the before/ after of my quick edit.
http://img905.imageshack.us/img905/8048/1nwKmp.jpg