Some good tips here and it's a hot topic in other related threads about plug choices and such. However, one assumption needs tested in ALL these various gas mileage claims; that when you refill the tank to measure what MPG you got, you refilled it to EXACTLY the same level. Because if you didn't then the whole basis of your MPG math is wrong. Maybe today's gas pumps are so consistently well metered that this is not a factor, anyone know? Till proven otherwise, unless you're using a dipstick on level ground to test before and after fuel levels to base your math on, it could be a factor.
Lets say in a 20 gal tank you drive 200 miles then refill it, at some other station and/or pump than you filled it up at (add variations in temperature to this also, a small factor). But at the refill you don't get it filled to the previous level, you're about 1/2 gal short of the fuel level you began at. So the pump trips off with a 10 gal refill and you figure you just got 20 mpg, but in fact, you used 10.5 gallons not 10. You think you got 21mpg but you really only got 20.
Was it windy when you drove? Aerodynamics are huge friction sources and our boxey KJ are not exactly built for low drag coefficients. That could give you a 1-2-3mpg variance easily.
Granted these are minor and don't account for say, going from 20mpg to 14mpg but they contribute.
If these 'tests' are done on the same strip of highway or city blocks in the same exact driving conditions and you refilled the tank the exact amount of gas used, then it's fact. Short of that it's a lot of speculation seems to me.
By the way, our gas mileage sucks too but I discovered it's my wife's foot. When I drive the car it goes about 50-75 miles further on a tank doing pretty much (again, not scientific..) kind of driving mix. As someone said, these are heavy beasts but we love our 2005 with almost 100K on it now, it's been rock solid.