Bedcovers?
Depends...Highway or city driving most? Highway I had one on my 06 Colorado and it was getting 1-2 more mpg. They're also great at keeping the sun, rain, and snow from getting on anything in your bed.
If I remember that episode correctly, they were testing tailgate up vs tailgate down, and found the gas mileage was better with it up by a good margin. This test did not include a bed cover so I don't know if it will help or not. I frequently put stuff in and out of the bed of my truck so a cover would just be a PITA.

