9mpg for that big of a beast isn't really all that bad....Been seeing 9 MPG so far running 65 MPH across I-84 through OR & ID. I'm feeling better now that I've got almost 1k miles on the new engine. And I'm sure the fuel mileage will improve as the new engine gets broken in.
Don't get your hopes up too fast.... It's taking forever for the mileage to creep back up with my Cummins. With out looking, I'm pretty sure I have around 20k on it and it's only back up to @14mpg empty, and that's with the canopy on. I was averaging 17 to 19 without a canopy before the rings on #6 cyl broke.