I wish Indy would start a new thread on this topic, so I could better assist him in getting a cold dose of reality regarding his MPG. As it is, here are some helpful talking points:
The single biggest factor for highway MPG is going to be wind resistance, not weight as he suggested in another thread. These rigs present a relatively large frontal area, and do almost nothing to mitigate wake turbulence.
He has mentioned he lifted his rig, which will make a difference. If he has a roof rack, that will make a surprisingly big difference. Similarly a light bar or other accessories hanging out in the wind like scoops. And if he has deleted his front air dam, he's just asking for it. And the bigger tires are costing him two ways: increased rolling resistance as well as wind resistance.
All these factors are going to be multiplied by how much faster than 60 mph he drives. Doesn't matter what the speed limit is, the wind resistance starts to go up quickly at 55-60 mph. All of it costs money.
I'm running stock tires, stock ride height. I take my roof rack crossbars off if I'm not using them. And I haven't added anything else to the exterior. When I bought my rig bone stock, with 36k miles, I was getting 17.x mpg all-around rural driving, measured in gallons at the pump.
Seeing how well I was doing, I started really laying on the gas pedal at starts, and especially at freeway on ramps, where I would let the roar out of the Hemi (purely in my quest for knowledge). As long as I kept the freeway speed at 60 (the speed limit) and DID NOT downshift for engine braking, it was difficult to get the mpg below 16.
I then added an aFe CAI (I know, I know, they do nothing). With the same driving pattern, it became difficult to get the MPG below 17.
Here is a profile for a typical 200-mile round trip to the mountains, where my mpg (calculated as gallons at the pump) has ALWAYS been 18.4 to 18.8:
Mostly flat, rural highway, 55 mph. Several little towns with 25-30 mph postings. When I hit the hills (about 30% of the driving time), it is nothing but twisty mountain highway with 35 the typical posting. I do much of this with my shifter set to 4 or even 3, and I make a point of letting the Hemi roar through the 15-45 mph range between turns, drifting the turns when I can.
Basically, 30% of the time I'm doing all I can to destroy the mpg, and I'm STILL getting in the upper 18s. What I'm NOT doing is using the rig as a giant parachute above 60 mph.
Hope this helps!