A great deal of the items that ricers use to improve performance of their cars is actually counterproductive (evident in the amount of black smoke that comes out of the tailpipe of these fart can mufflers). If you have holes on the back of your bumper, you would actually encourage more airflow to your rear bumper and create a drag. I would be curious to see this theory tested in a wind tunnel. I imagine that the rear bumper area would do the same as a pick up bed only upside down. Instead of drilling holes, why not just remove the bumper?
