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My 2007 has the Allison so you have the advantage with 12 gears versus my 6 but over the last 11,222.7 miles I have averaged 5.5 mpg according to my silverleaf.  The best mileage I ever achieved was when I was going to Key West, FL, and averaged 7 mpg but I was just cruising along at about 45 mph for about 100 miles and the only hills were bridges.  I normally travel no faster than 65 mph.