Sorry for this but I need help with my 1992 K3500

Let's start with "how are you" figuring mileage? Are you using the speedo? If so, trash that idea, and go buy yourself a GPS. I have picked up about 3 or 4, less than 40 bucks apiece, on CL and the thrift stores.

These make excellent accurate speedometers and odometers.

ALSO you need to be honest and define your driving. Is this "open highway" driving, or a mix of "in town" hills, etc? How big is your right foot, figuratively speaking?