I get 14-15 mpg on a good day in town.If I drive around 67-69 mph I can get 19-21 mpg on highway. Thats with MAC air filter, CAI, and that thing called an air razor. I'm not for sure if those air razors actually work, but it can came with the truck so I just left it alone. Here's what it looks like for those not familar with it...

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Do you have 3.55 or 3.92? City driving kills the mpg, esp on a 4x4. I had the same issue with mine dropping to 10-11 the first couple thousand miles. Pen and paper works best. For all the non math people take the number of miles you drove from your last fill up and divide by the amount of gallons it took to fill up. Some geniuses say to run it till its almost empty and use the tank capacity listed to determine it, its a lot easier (and smarter, may just wind up on the side of the road with a red can in your hand) just taking how many gallons you filled up with be it 10 or 25, will get the same result. Try not to keep it over 2k rpm for the most part on highway driving, for me thats about 72 mph and 16.5 mpg, surprisingly at 90 i get 13 (would have thought it would have been less). And lastly lets not forget about the idiot factor, you do have the O/D turned on and not the tow/haul mode on right???? OH and if you want a few more hp and slightly better mpg, try adding a throttle body spacer in it. Also a K&N air filter, I'm a big fan cause in live in south texas (aka desert) so reg filters need to be replaced often where as once a month i just wash mine thus insuring clean and smooth air flow.
Oh yeah, i swear to god idling kills it, I reset my mpg gauge and for about 1k miles i was at 15, i sat a drivethrough for about 20 minutes and it was down to 13.8