They don't like wind.
On a still day you'll bump 22mpg and loaded you'll see 15-16. All ya gotta do is stay out of that turbo.
Yesterday, I had to haul my big tractor to town to have repairs done. After I kicked the duals off I got the tractor weight down to 14K pounds.
Truck didn't even know it was back there. Oh, and you're gonna love that engine brake. :>)