It is difficult to believe that a Tahoe 350 can get 18mpg in the city. I get 14mpg out of mine, and I consider myself lucky, as I know many people in the 10-12 range. I only get 18mpg out of my Jeep with an I-6, and about 10mpg out of my CJ-5 with a 350.
I drive with a light foot, only city driving, stock 97 on 285/75's, and I always get at least 380 miles to the tank. When I fill up, it takes 26-27 gallons(cost: $100 and increasing), which I have to do three times a month.
I think for the size and power, the Tahoe is good on mileage. I feel safe driving mine, moreso than any other vehicle I've owned/driven.