Yes its a super duty. I've gone 175 miles and the needle is just above the 1/4 tank mark. This is with a 20 gallon tank, 3 inch suspension lift and 285's.
Is that bad for this truck??

 

The needle is not a useful comparison for gas mileage. There are only 2 ways to measure fuel economy. 1st; Empty t he feul system and pour in a measured amount of fuel and drive till it stalls out. 2nd, my fav, is to fill up the tank till it reaches the brim and drive. When you have driven far enough fill it up again to the brim and calculate from there.

JMC

 

Actually here is the story from Mythbusters....

It is more fuel efficient to drive your pick-up truck with its tailgate down, rather than up.

busted

Driving with the tailgate down actually increased drag on the pick-up and caused it to consume fuel faster than the identical truck driven with the tailgate up. It was later revealed that the closed tailgate creates a locked vortex flow that created a smoother flow of air over the truck. With the tailgate down the trapped vortex was dissipated and the drag increased.

(This myth was revisited in episode 64 and re-busted. However, it was found that mesh tailgates are the most efficient configuration.)