I have a '97 3500 4x4 stock and have seen 20 twice @72mph,19 twice but normal is 18-18. 5 empty @72. 75+ it drops considerable. Towing 29'Terry 5th & 17. 5'Crestliner on double axle trailer 9-11mpg. Either 5th or boat alone 15mpg. Montana mountain driving. Must be in the ball-park with the rest!