I have a 2006 LBZ in a GMC, crew cab, shortbed, 4wd, Allision trans. Bought new in April 2006. I have recored the following mileage:
Tank #1, actual 18.6, DIC says 16.9
Tank #2, actual 19.6, DIC says 17.5
Tank #3, actual 19.8, DIC says 17.6
Tank #4, actual 18.9, DIC says 17.3
Tank #5, actual 18.9, DIC says 17.4

All driving is combination city and highway with AC on, highway speed 65 mph. Truck is 100% stock. Using Stanadyne Performance additive in every tank, 6 oz.

I have asked my Dealer if the DIC is always inaccurate, he says they have found that it varies in every vehicle, some are correct, some are low and some are high on the mileage calculation. I have asked them to contact GM about it. I would think that there must be a calibration adjustment somewhere. If not then the DIC info is of little value in my opinion.

Your comments,
JimO