I too have an ISB 275, Presently in Idaho,Just figured my MPG, Boise to Coeur d Alene, Some very steep hills, lots of exhaust brake and keypad shifting for the Allison, 10.1 towing Jeep Grand Cherokee.
Not speedy up hills, but very strong and cool.
Terry.
Monaco Diplomat, Cummins, 38a
Toad Grand Cherokee and Jeep Wrangler, Brake Buddy.