[AZKen]

As said, we don't know your usage, loading, terrain and possible Holley carb internals. Even with a 305, that's pretty low MPG. Unknown history from PO. If you 100% positive about ratio, great, but could have 3.92 real easily. You can't do accurate ratio calculation by tire/shaft rotation.
You may be falling into the category of "playing with it too much", "modifying" too much. Original factory motor Synergy gets out of whack. Go back and try a 350 CFM original carb. You could be wasting gas.