My '96 5 speed 4. 10, gearvendors OD 2wd 3500 ex cab w/ camper mirror swung out gets 19. 5 daily, and up to 21. 5 on long trips at 70 - 73 mph. 10 - 13mpg towing 20,000 gross. A high Swiss topper did not change it one way or the other. My timing is stock, 13 deg, I think. I might try 16 deg, that would help.
For the weight and size of our trucks, they do very well, almost twice as good as a v10 gasser.
Doug Rees