That is a large, heavy truck, and will have a 4L80 transmission. Properly built the 4L80 is nearly indestructible, but causes increased parasitic losses due to the power required to turn it. As you already know, the truck will never be referred to as fuel efficient.

Take a look at the threads regarding Lean Cruise and DFCO (Deceleration Fuel Cut Off) settings. You should be able to squeeze another 1 to 3 MPG in efficiency with some tuning.


Regards,
Taz