I have not checked mileage in 35 years on any of my vehicles. People used to check it to brag about it. If truck runs good, I don't see a problem to be solved. I do see a problem to get initiated by screwing around with things. The terrain of each trip, the weather (temp/humidity), the altitude, the gas, the filters, air cleaner, spark plugs, your foot, your math, tire pressure, drive train drag, brake drag, Mother Nature and Father Time may be the cause. As well as the carb and yes, oil viscosity.
