I have 350 chevy engine in 1965 Chevelle, 4.11 rear, 200R4 trans. Tire is 26D. At 75 MPH it gets 15-17 MPG at 2700 RPM. I am thinking about gear change to help milage. With a 3.08 gear it would turn 2000 RPM. Anybody have ideas on posible milage increase?