Please specify if you're driving a tahoe/yukon or suburban type truck. As the suburbans do have larger gas tanks than tahoes. Short wheelbase trucks (both 2 and 4 door) get a 26 (what GM calls 30) gallon capacity tank, while the suburbans received a 42 gallon tank.
I get around 250, all city, 26 gal tank.
If this is true, then I'm not sure how I managed to squeeze in 29.8 gallons when I ran my tank dry years ago...
Usually in the summer I start looking for gas around 300 miles, though normally I can get 450 out of it before I'm getting nervous. I like to keep fuel in the tank because the pumps on these are not known for their reliability and they use fuel as a way to keep cool, so if you urn it low on fuel it'll get hot and fail sooner.
Once, because I had the time and wanted to see, I took some backroads and set the cruise at 55 mph and drove to Utah from Washington and almost did it on a tank. That was 575 miles. It pays to go slow, as that netted me nearly 20 mpgs.
Normal driving (mostly highway) I net 16.1 with the a/c cranking.
i honestly dont see it either i haavent checked in a while but im pretty sure i get maybe 225 250 on a full tank no bs how the same trucks are getting almost 400 or more bewilders me i dont drive like a pissed off teenager either thats for my daytona 
