Is my 340 Overcarbed? Avg 5-6 MPG

Before Al Gore invented the internet, and before we had this fancy GPS stuff, I used to just fiddle until I had 60 seconds per mile on the marker posts. "Correct" the speedo from there However speedo error and odometer error are not necessarily the same thing in all cases

Before we had these fancy online tire charts and speed / gear/ mph calculators, I've been known to actually measure around my tires and "do the math"