With my drivetrain and the weight of the vehicle...the engine pulled about 15 inches of vacuum at 55 mph. At 75 mph it pulled 20. The engine was running more effectually at higher speed. If you do not have a vacuum gauge hooked up you can hear when your engine is running its best...it gets quieter. Air/Fuel ratio also plays a big part in mpg. So does tire air pressure and carrying around dead weight. All vehicle are different...your mileage may vary, it is possible that you will get better mileage at 65 than 75
Mine gets around 16 on the freeway. It's an old 302 with a bigish cam ported big valve Windsors, 3.70s and a T-5. Likes to cruise in the 65-70 MPH range. Oh and a Holley 650 DP (Probably the reason it doesn't get a few more MPG).
