I run a 3 inch level (which is essentially a 1 inch lift) and 315x70s (which are essentially 34s). For me the mileage difference with the level and tires was virtually negligible. It was once I removed the front fascia where my mileage plummeted.
Note that I run midgrade gas. The tildes (~) are the EVIC's calculated average - the rest were manually calculated by averaging 3 fillups.
My truck used to have 3.55 gears before I switched the front diff. The transmission shifts fine until its under load (interstate speeds or towing), in which case I just leave it in 4th.
With 4.11 gears I'm at about 1800rpm in 6th @ 60mph, and ~3000rpm in 4th @ 75mph, which keeps the engine in its happy place to maintain speed without hunting gears regardless of load.
What I couldn't stand, and what made me swap the front diff, was the offroad performance - the 65RFE has a somewhat tall 1st gear, so even in 4LO you can't really crawl. Many of the trails in my area are narrow, have steep descents, and are shared by ATVs - so without good gearing you'll be flying down these hills, and forced to ride the brakes the whole time. Adding larger tires does not help matters. Switching to 4.11 helped a bit, but I think be much more happier with a 4:1 transfer case.