For starters you need to hand calculate to get accurate numbers.I have an 2007.5 it has 6" lift with 35" tires. The truck is all stock and gets 10.7 mpg according to the on board computer. What gives?
You were not getting almost 21mpg.Believe it or not my '03 lb7 got 20.6 mpg between Houston and Dallas. 75 mph avg. On 38" tires, Edge juice w/ attitude set at level 3.Engine turns 2000 rpm's @ 75mph.
Remember a 6 speed drops rpm by 300.How are you getting that low of an rpm with your truck. I run 2200 at 75mph