Here is a test done by a guy to find out if tailgate down improves mileage
Tailgate Up/Down and Gas Mileage
By ron.delong | Published: May 3, 2004
http://www.scangauge.com/tips-and-tricks/tailgate-updown-and-gas-mileage/"The test was started when I passed milepost 15 and completed at milepost 20. I then went back the other direction pressing RESET at milepost 20 and noting the trip MPG at milepost 15.
The average of these two was taken for each condition, tailgate up, and tailgate down.
The results were surprising.
TAILGATE UP = 17.6 MPG
TAILGATE DOWN = 17.7 MPG
By surprising, I also mean disappointing.
Did the mileage go up with the tailgate down? Yes, but the amount is so small as to be statistically insignificant. A single gust of wind during one of the 10 mile runs could account
for this much difference."