I had an 03 Duramax/Allison with a 4" exhaust and a Quadzilla S2 programmer. I knocked down upwards of 21 empty and 18 loaded. Loved that truck.

Are you sure it's a 4spd auto? I was unaware of any Duramax trucks having anything other than either an Allison auto or a Z6 manual.

No, I stand corrected its an allison 5 spd automatic

You should get around 16-17 in mixed driving empty and towing will depend on how big. I get 12 mpg towing a 28ft TT at 70 mph. The only reason I avoided the earlier Dmax (pre-04 1/2) was the injector probs some of them had. The 6 speed tranny gets about 1-2 mpg better than the 5 speed empty and almost none better towing heavy.

I have an 07 1 ton with the dm and allison. Great truck, it pulll anything you hook up to it with ease. I get about 15mpg whether I am towing my boat or not.

I have an '03 Duramax, extended cab, long wheel base, 5 Speed Allision, 4 wheel drive, 305/70/16 tires. The only modification I have is an ATS programmer. I average 17-18 on the highway and 15-16 pulling the boat.

Thanks for the info Monty! Mine is 03 ext cab, long wheel base 2wd, and it has a leveling kit sitting on 285/75r16. I went to vernon this past weekend and got around 20 going up there and fought a hellacious head wind on the way back and might have got 15 or 16 and that was with the cruise set at 65 both ways