my 2cents: I have a 2001 Tundra and I'm getting ~19.6 to 19.8 mpg mixed hwy/city. I attribute that to two things, 1. Driving like an old lady
2. Having a topper shell(level w/roof) on the back. Not crazy about the way they look, but sure helps keep the down draft off the rear end.
Plus, keeping my ride maintained properly, of course.