Best I have ever done was 14.4 on a road trip to Florida last summer, I drove the whole tank and averaged around 75mph (my wife gets horrible gas mileage when she drives.... leadfoot and not steady with the throttle). Im sure if you took a highway trip and kept it around 60-65 you could see 15-16mpg but you will get your ass run off the road at those speeds on just about every highway in the U.S....... In the summer with easy mixed driving I can get 12+ but for the most part Im in the 11's (In the really cold weather with warmups Im barely over 10). Firgure an extra 1-2mpg's with the 08 up H2's with the 6 speed trans. The H2 is very heavy, is on a 3/4 ton chassis and is shaped like a brick so there is not much to do other than drive downhill if your looking for good mileage.
Sorry that isnt the exact answer you were looking for but its the cold hard facts