I seen a study from U of M's website of the 99 style GM truck clay model in a wind tunnel, and they concluded that having a tonneau cover was the best, and having the tailgate off was the worst. I just installed my Pace Edwards cover, no conclusions yet as I'm still on the same tank.

 

TRY THIS. Coast down times. The Nextel Cup boys have come up with a low buck way to measure drag. An old airstrip is the best place to do it, but a deserted road will work. Use time or distance and push in the clutch/ neutral the trans at the same point and speed and see how far/long it rolls to a stop. Back-to-back-to-back gate up-down-up and see?

Here's another one for the chassis dyno. Click it off at an RPM and see how long it coasts on the rollers. Things like driveshaft angles have an effect. One thing fould: Having a zero angle is less effificient than a slight (1 degree) angle in a U joint. I am guessing it has to do with an oscillation set up when the U joint is straight, that does not happen with a slight angle.

By cranking down on the chassis with ratchet ties, you may find the lowered vehicle has differentdrag from the static chassis heights. This could be done while making power pulls, also.

 

If you decide to go with the tailgate down, get a bar or something to secure the bed walls. They will vibrate, causing the seals to crack(maybe even some welds) and cause it to rust out.

 

I have a 110 mile a day work commute and put 66000 miles on a '01 DC 235/460 auto 3.54 gears and check tail gate down vs up lots of miles and tanks of fuel, it made no difference. All 75mph interstate. My'03 DC 305/555 6spd 3.73 gears has 52000 miles and checking tank after tank on the same drive shows no change in mpg. I bought a new '77 Toyota and put 139000 miles on it. Drove the same 110 mile commute and if there was a lot of wind leaving the tailgate down would get 1 to 1.5 better mpg. When there was no wind it didn't make a dif. .......JIM