UPDATE:

I added a soft/folding tonneau cover last fall, mostly just to keep the bed clean and the ability to put stuff in back and keep it out of sight/locked tailgate. I haven't watched my fuel mileage that close lately as it's been SO COLD out here that I'm spending a lot of time letting my Ram idle outside to warm up, so my mileage hasn't been great day-to-day.

That said, on Monday and Tuesday I had to drive (for work) over 140 miles each day, all interstate, and decided to check my mileage. So, I filled up at home, drove this trek two days in a row, and filled up at home again. I clocked a 21.9mpg trip with my 2011 Ram 1500 Quad Cab 5.7L Hemi 4x4. I was impressed! I never got that kind of mileage before adding the cover. Now, I don't want to say that the cover did this...but, I was NOT driving these two days in any way to increase gas mileage. 65mph zone, I was treking in at 75-80 most of the time trying to get there quicker...

I'm very happy with the bed cover, and it's certainly not HURTING my fuel mileage!

Thanks for all the replies! I really think that it helps, I don't really know how much, but every little bit counts at these gas prices and the amount of miles that I drive.

JS