I own a 2001 4x4 Limited and get roughly 13mpg in urban driving and 18mpg on the highway. I usually put 22 gallons in the tank.
As for the tires, I run 32~35 PSI all the way around.
Don't feel too bad about the gas mileage. I have two co-workers, one drives a fairly new Tahoe and the other a late 90's v-8 Explorer, and both report averaging 11~13mpg on the highway!
