lets revisit timing

Fuel, compression ratio, chamber design, cam design and cam timing, AND WEIGHT AND USE of the vehicle all play a part

The "correct" answer for fuel mileage has always been "vacuum advance."

"Back in the day" with "real" fuel and with a factory "high compression" 340 or 440 I used to run as much as 38-40. But approach that carefully, and even though the plugs did not show it that " I could see" one 440 ate a couple of top rings/ ring lands