You guys need to realize a couple things.
1) Looks like the OP has no weight carrying mods
2) Mostly freeway driving
3) He goes slow, he is the one you people get mad at for going under the speed limit
Now from the posts I have read, everyones got lifts, tires, ect and have commutes (from my perspective) that are not all freeway driving. So with all the additional mods and city driving you all do, its simply impossible to achieve his numbers.
And I also agree with the rest of the people that say post up the MPG not the miles per tank...who knows, for all we know you could be driving the tank to bone dry and filling her up with 20-20.5 gallons which would make your MPG 20, 19.5 instead of filling up right around the light 16-17 gallons, 25-23.5 MPG.