I tried running the tailgate down myself just to see- and my MPG did get worse. I got a tonneau shortly after and checked again- I saw a 1.5mpg increase.

I found that driving habits were the biggest gain (and they don't cost anything). If I drive my stock mega cab 6spd at 65mph or less, and shift before 2Krpm, I can get between 19-21mpg city/hwy combined out of my totally stock truck (only mods are tonneau, bedliner, nerf bars). At my best, on the upper half of a tank, I got 23.8mpg. That was 75/20% HWY/City with an average speed around 55. All hand-calculated, the computer is always 1-2mpg high.

I bought my truck new- and saw a huge increase in fuel economy at 15K miles (before that the mileage was all over the map anyway) when I bought and installed a new fuel filter. Now I change the fuel filter when I change the oil. It's $9 and takes 5 minutes- definitely worth it.

I'm due for an oil change and am going to switch to synthetic- I have a friend with an 01 that saw a 1.5mpg increase on his when he switched to Royal Purple- though he already had over 100,000 miles on his.

 

it was on myth busters,they did it with gate up and down and stated best results were with a tailgatenet.im sure a tonnau would do the same too.