I've got 4.56 gears in my 3/4 ton and I can comfortably drive at 65, which is about 3000 rpm. I can go faster, I've gone up to 75, even 80, which is around 3500 rpm, but the vibrations from the driveline are so bad that I tend only to use that speed for passing in the left lane.
If I switch to a 3.73 rear end, (if I buy my friend's posi Dana 60), my top speed would probably be almost 90, but unless I fix my driveline vibe, I probably still won't be able to exceed 65!
