CM871 fuel mileage

[SquareOne]

What are your driving habits? Whats the trans ratio as well as the rear ratio? Are you keeping the boost 10-12lbs and lower most of the time? Are you lugging the engine down or are you letting it turn up 16-1700+rpm between shifts while loaded or climbing?

All these factors will greatly affect fuel mileage.

What do you haul? Type of trailer? How much deadheading?

We need much more info to give an accurate answer. At first thought, yes 5mpg with 70k does seem very bad unless you're pulling some ridiculously non-aero load.