according to this study the tonneau cover is the best reduction in drag.
Check this out it lists drag Coefficients for different configurations...
http://mars.acnet.wnec.edu/~ehaffner/did.htm
<snip snip>
>Consumer reports did
>a long term test a couple years ago and claimed that it lowered mileage
>as
>well with the TG down. And I've heard reports that at highway speeds in
>the
>rain with the tailgate down, without the separated air bubble effect you
>run a
>much higher risk of having the rear wheels hydroplane on you becaus it
>actually has a lifting effect.
<snip snip>
=====
Nate, Ogden UT
http://www.geocities.com/natedak2k
Forest Green 00 Dakota CC 4X4, 5-speed, LSD, 3.92 Rear, 4.7L/287ci, Po'boy cold air w/ K&N, Extang BlackMax Tonneau
"D2K"
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