Tall tires change your gears too, so that also slows you down when accelerating. That's why so many people with 35s or better look forward to aftermarket gears. If you have the Big Tow or Off Road pkg, gears which are lower it isn't as bad as if not.
Theoretically the taller tires and poorer performance should give you better fuel economy, but as VT21 said the extra weight of the tire will hurt due to the rotational mass. Worse for city or stop and go because the heavier tires ... once going, want to keep going ... flywheel affect.
Some tires are worse too. Like the Toyo and Nitto tires, (heavier), compared to the BFG or Kumho tires. Supposedly each extra pound of rotor, wheel, or tire is equivalent to 10 extra pounds in the truck's bed.
I lost 1 mpg going from OEM P285/70/17 43 or 44 pounders to an aftermarket LT285/70/17 at 58lbs. Come to think about it, it does seem a bit more gutless, and the size is basically the same, so mostly rotational mass if something else hasn't changed.