I agree with the paint damage. I tried driving with the gate down ( on my old Chev 1 ton dually) only for a few hundred miles, and there was noticiable sand-blasting damage.This was all pavement driving on the hiway. Not only was there no improvement in mpg, I had a couple of hundred $ damage to tailgate. I learned the hard way. Also the tailgate is part of the structure of the body,it helps keep the box sides inline, over time driving with no gate or gate down will likely lead to a skewed box.
Maybe try a flow-thru gate for better mpg with out doing any damage?
My opinion,only.
Mark

 

After day 3 (last Wednesday) some a-hole almost rear ended me, so I removed my tailgate. Tis better for them to hit my wrap around pipe bumper than my tailgate.

I averaged my last 6 tanks and show 17.2 mpg.

I filled up Monday morning (5 days, 480 miles). Averaged 17.6 mpg. I find this to be very good considering last week (and this week!) we are having unseasonable high steady winds at 25-35 mph. Last time I drove through winds like that (Thanksgiving) I averaged 15.4 mpg.

I will run the rest of the week and fill up again Friday (need to tow my trailer this weekend).

For comparison, last Wednesday I drove 10 miles with my tail up, then finished with my tailgate down. Definetly a "seat-of-the-pants" difference.