I run around 20 mpg empty, and fully loaded with the lowest about 16-17 mpg. Then again with my 4.10s and C6 I run 60 tops on the highway. It ****** people off, but I enjoy the ride of a 4+ ton beast while watching all of the plastic death traps zip by. The tires are straight out of ""Hot Rod Lincoln" and are 265/75-16s with lugs so I can pull the 32' gooseneck loaded through the fields loaded with hay, or broken down tractors - whatever.
There's no kickdown on the tranny, the modulator is crap so she shifts a little hard, but the truck is a gem to me no matter how rough it looks. I double checked the speedo with a Garmin Rhino and Im pretty much right on. Though in my current project 88 I want to go with 3.55s since it's not going to be pulling the gooseneck, just off the frame mount receiver hitch in back.
Of course I do throw in a few additives every now and then. Every tank throughout the year gets at least one bottle of isopropyl, Howes treatment when I can afford the extra cost, and at least some 2 cycle oil, ATF, and some waste oil in most tank fills. It likes the Howes, the engine seems to quiet down a little, and the 2 cycle adds to the feel of the skinny pedal. But I'm running shy of the synthetic 2 cycle, may actually have to buy some at some point.
I run synthetic oil in everything but the engine crankcase. Why not there? Because it already has a crush on the Mobil Delvac 15w40 I feed it year round. The engine I just picked up I may try synthetic in since it's got better gaskets on the valve covers and belly pan. We'll see.