I drive a 99 Dodge 3500 Dually with auto transmission, the 24V 5.9 Cummins. Best mileage I see is in town with careful driving, about 15-16 mpg. Towing a 5,000lb trailer at 60mph I get 12 mpg. No overdrive. Given that I have the somewhat frail 47RE transmission, it seems foolish to try and pull that load in overdrive.