I consider myself still a newbie. Had to take my rig over the other side of Houston today. First time I have run complete interstate as opposed to back road, 2lane rough roads. I have a Scangauge hooked up to monitor my real time mileage. Based on my first tank mileage, i was very displeased with myself - only averaging 6.2mpg. I learned quickly to slow down(65 or less)...getting better real time mpg equated to a better run at 7.6mpg on my last tank. Today I was averaging about 8.5 before filling up(last tank was split across trips). After filling up I got caught in slower Houston traffic - about 55ish...mpg was even better. In fact the average was 9.8 when I pulled into my dealer's parking lot about 50 miles down the road.
Some observations....smooth interstate roads perform better than the 2-lanes; finding an 18 wheeler to hide behind creates some interesting drafts - smooth and seems to pull you along....mpg increases here too.
And yes - I took have noticed my Scangauge goto 9999 when coasting with no throttle.