Havent done a recalibration yet, but this did cross my mind as a possibility.OK - Maybe this is a dumb question on my part, but did you recalibrate the speedometer after switching to the 34's?
As I understand it, the truck is actually counting revolutions and translating that to distance. If it thinks you have 33" tires but you now have 34" tires, it will calculate fewer miles than you've actually driven. Therefore, the truck will display ~3% worse miles/gallon than you are actually getting. The heavier mass of the bigger wheels and the worse aerodynamics from the lift are bigger factors.