Yep, that's spot-on for me with my 4.3L. I think a lot of people who brag about high MPG are being a little dishonest with themselves and maybe not telling the whole story. When I calculate my MPG by hand, I usually get around 18 if I've been driving economically, and I've seen as low as 14-15 if I've been driving REALLY aggressively (I live in Socal, our freeways are a free-for-all deathmatch).
Yeah same here, the computer is waaay off on mine. And I'm running stock tires with no lift. Computer is off by as much as 3-4 mpg sometimes.