Some random thoughts.
My take on the D.I.C.: (Don't) Trust, but verify. Mine is seldomly better than .5 MPG off.
My MPG: 15 - 17, with my best now being 18. (2005 C/C, L/B, 2WD, all stock - except tires - See below)
ULSD: The change-over was before I owned my LLY. But I know that the Cat 3126 in my M/H lost 2 - 4 MPG due to ULSD.
Tire size vs. MPG: I lost 2 - 3 MPG due to the 265/75-16s I installed on my truck. With the 245s, I always got 17 - 19, with the best being 20.8.
'DPS' in Van Nuys, Ca. recommends the 'Predator' hand held programmer that can install/uninstall different tuning maps for power - economy - etc. Customer claims of 1 - 3 MPG.
Regarding above: I'm chicken. Too many horror stories of codes being flashed due to re-programming. (In all fairness, very few ugly stories are about the Predator - But I'm still too anal to experiment)
Dan