6 banger mileage?

Mine gets 20 city 28 highway @80mph with 2.76 gears, 175,325 miles as of this morning...

Calculators say with a 25.3" tire (205/70/R14's), 68 mph puts me at 2,500 rpm. 81mph puts me at 3,000 rpm... Imagine the fuel economy I'd get if I had an overdrive gear...