The 6.0 gassers on the GM trucks get terrible mileage.
Here's my experience:
I think my Jeep gets around 14-16 MPG w/33s and 4" lift. I haven't been able to calculate an accurate tank yet this year, and the prior calculations were corrected for speedo inaccuracies. Most of it's life has been offroad last year as well. Best I ever got was 20 with the hard top on and stock 30" tires, all freeway and a severe hangover.
I had a F-250 EC/LB 4x4 gasser. 460 CI big block. It got 8 mpg when I bought it. 10 after I tuned it up and liberated the cat. I forget what gears it had, but it had a C6 trans, so no overdrive. It turned about 3000 RPM on the freeway.
No muffler either. It was obnoxious.
It got 10 whether I was hauling or empty because it was such a beast.
After that, I bought the red half-ton above. EC/SB 4x4. That had the L33 5.3 and 4L60E. Super engine. Stupid trans. It had the some-what rare 4.10 gears. 100% stock. If I really babied it, I could get almost 20 MPG going 55 MPH. I could get like 17 or 18 on the freeway. Real world, I got 15 or 16. Towing the Jeep dropped to 11. The MPGs calculated on the dash display were always within a few tenths of my hand calc.
Bring on the diesel. Also EC/SB 4x4. LBZ duramax / 6 speed Allison tranny (best combo every built, IMO). 3.73s, MBRP exhaust and the cat went missing on this one too. It's lifted a tiny bit, and has the 33s on it. I got over 20 MPG (22? I don't remember exactly) driving on the freeway when I brought it home (65-70 MPH). Since I've been working on it, there's been alot of stop and go, and idling. I've only run one tank with decent driving on it, mostly around town, and it got 19 hand calc. (This one is about 1 MPG off on the dash display.) I haven't had a chance to tow with it yet.
That's why diesels kick ***. Better power and better mileage. Downsides are the fuel costs a bit more and (proper) maintenance is expensive.