I had a 41' 2007 Mandalay with a 450hp Cummins diesel that got slightly more than 9 mpg. I now have a smaller 34' 2013 Excursion with a 300hp Cummins diesel that has been averaging 10.5mpg since I bought it, which was a pleasant surprise. I hadn't expected to do that good. However, how much is due to the difference in wt and hp vs a more efficient engine, I cannot say.