While the engine produces maximum horsepower at 3,400 rpm, it produces maximum torque at 2,000. If you are wanting to cruise at 65-75mph, you will want a gear/tire combination that will do that at 2,000 rpm or just a little over, 2,400 at the most. To run extended time much higher than that will pound your 6.2 to death. These engines were beautiful for what they were designed for, but rpm was never their intended design. That said, they scream rpm wise for a diesel in their era. Cummins was not out yet, but they typically were governed below 2650 in the early years. The Powerstroke was not much higher. I think the civilian 6.2 engines are governed lower that their military counterparts. If you can hit the 2000-2400 rpm slot, you will probably see your best fuel economy. The 6.2 has an extremely high compression ratio and an unusually small rod for the application.