The 1/2 ton's from that year were rated at 11/15 city/hwy stock. The 3/4 ton's would have probably been just a little less, say 10/14, add the lift and big tires, and you are probably not too far off for city driving. I would expect on the highway you would get 11-12, but not much more. A regear would help get some of your economy back, and would make your speedo accurate again.
Cobra CB, Dual antennas, Dodge molded splash guards, Tekonsha P2 brake controller, Weatherguard tool box, AirLift 1000 helpers springs, Line-X bedliner, B-Pillar tape removed, 35% tint front windows.