You have any cites for that information or where did you get this opinions from? I would be really interested to see it or know what your statement is based on, at least, just for interest sake.
I have been told this by pickup owners for a long time and it makes sense to some degree but not some of the radical numbers I have been told.
I only drive mine with a cover on, unless loaded to the top, and have never cared about economy. It was a Chevy POS bought to do a job and sits idle now. I never drove it over the speed of sound so I know the tail shape becomes important to wind resistance.
---------- > Gas economy. I have been told without a cover and with the economy goes