What is typical fuel mileage for newer diesel semi tractor-trailer rigs on the highway?

Between six and twelve MPG, depending on terrain, load and a bunch of other factors. Drivers who have good trucks like Volvo VN780s, International ProStars or Freightliner Cascadias, and who drive them intelligently (accelerating smoothly, for instance) don't have much of a problem getting 8mpg on a consistent basis. I know two drivers with Kenworth T660s. One of them gets 12mpg all the time. The other works really hard to get 7mpg. The difference? Terrain and load. The high-mileage driver pulls loads of tires, which are fairly light because they're hollow, across level terrain in Texas, Oklahoma and Louisiana. The low-mileage driver pulls loads of paper and beer through the Appalachians and the Ozarks.