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My mom has one in her Yukon XL, it's averaging 11.2 MPG. None of us that drive it are very easy on it. No upgrades yet, but this thing is pretty quick stock. /forums/images/graemlins/thumb.gif
I've got an '03 Sierra 2500HD crew cab with an 8.1 and the six speed manual. I get 14.5 to 15 mpg on the highway at 70-75 mph and the a/c on. The only thing I've added is a K&N filter. I also run Mobil 1 oil and a PF-1218 filter.