I"ll keep this short as its a little off topic for the tread. 16 mpg sounds good for a new truck. I have a friend with a new 2500 long box quad that is getting 19-20 if he keeps it under 70mph. Above that it noise dives real quick. Around town he's doing 16 or so. If your overhead computer has the mileage as you drive function, keeps changing as you drive second to second, play around in differant gears at differant speeds and see where its indicating the best mileage. My best mileage towing my Carriage is at 55mph in 5th gear or 13.5 mpg indicated. Its not really that good but it's a high 12 low 13 when calculated. With the HP rating of my truck stock, which is still is, I would do better with a 4:10 in overdrive. It would be like 5th gear at 55 only at 65. A small HP chip or programer would help as well.
I personally would not chip you cummins until its broke in good. Some say 25-30k. Don't really know. You mileage should get better as it breaks in.
Did almost 30 in the Coast Guard, one over in Nam 68-69. Thanks for your help in Iraq.
Thanks Rusty for letting us get off topic a little.