[jhv48]

Unless you can cruise at 80mph on your roads, the .64 will be an overdrive. There's about a 1000 rpm drop from 4-5. I, personally, don't like to cruise at 1500 rpm. My engine starts being happy above 2000 rpm.

I had the .64 with my 3.46 rear end and was turning about 2200 at 80mph. Acceleration in fifth was almost useless. Had to downshift to fourth for any spirited passing.

Changed to a .82 fifth gear and now I have a useful fifth gear. Turning 2600 rpm at 70mph. Want to pass, just press down on the pedal. No need to row back and forth between fourth and fifth when traffic slows.