going to hunt for new ring and pinion gears

[dbenamati]

I talked to a mechanic today and told him of how slow my truck drives and high rpms. It's a 67 3/4 ton, 4 spd. that tops out at 55 mpg. I don't know what gears I have but he suggested that if I search the bone yards for 3.73 ring and pinion, he could change it out and I could have up to 70 mpg's with lower RPM's. Anyone done this and if so, what gears could I use? I just want to move along with traffic, not break the sound barrier

Thanks, Dave