Hi, I am new to forum

I purchased an 2007 LMM six weeks ago and mileage is as 5780 miles-404.1 gal. 14.3 manual calculation DIC says 14.7. I drive mostly highway unloaded and drive like an old granny. 60-65 MPH. Needless to say I am not happy with mileage. I have had diesel PU's since GMC started with the 5.7 and have had Fords, Dodges and GMC's.The Dodge was a fairly good truck but the dealer was very bad so no more Dodges. Currently in my business I have 4 diesels 2001 GMC 2004.5 GMC, 2005 F350 6.0 Powerjoke and the latest 2007.5 and this one is currently the least efficient.
I have the Bully Dog Pup on my desk right now and need to download. I am hoping that this will give me a couple MPG.