I do 400 miles of highway driving a week @ about 70 mph with 3.73, the older 4.0L OHV and 33x10.5 tires. Economy is as low as 14.2 if don't care, and and all time high was 16.2 miles per gallon when I really pay attention.
Course, my truck is pretty tall, has 200+ lbs of steel/bumper/winch hanging off the nose, and at least another 200lbs of extra weight due to rock sliders and aftermarket lift kit parts.
I wouldn't be surprised if a slightly lifted truck with the newer SOHC, 3.73's, and 33x10.5's got around low 15's to mid 17's in terms of fuel mileage (such as your truck).
Width of the tire will also make a big difference, 10.5's contact patch is smaller, tire is lighter, and less backspacing all helps elevate potential mileage numbers. Whereas 12.5s have a pretty big contact patch, are heavier, and more backspacing to set the tires out farther from the body. Expect around 14-17 with 33x12.5's.
I chose 33x10.5 for 5 reasons:
I like the look of a skinny tire
Very good traction in mud, ice and rock
Better fuel economy
Less wear and tear on drivetrain due to lighter tire weight
Skinny tires = a narrower track width due to wheel backspacing (means I can get in some really tight trails
True 6" lift, SJ brackets-coils-leafs, JD RBV/EB Buckets, Bilstein 5125's, Lock-Rite Front, No-Slip Rear, 1-ton SuperRunner Steering, TJM T13 bumper/winch, 10" LED, 33x10.5 KM2's, Bolt on Thread http://www.therangerstation.com/foru...ad.php?t=75235