To calculate RPM at a given speed the formula is:
Gear ratio X Transmission ratio X 336 X MPH / Effective tire diameter = RPM
"Effective" tire dia. is the true rolling diameter under a load. A 33" tire is not really 33" tall when loaded and it can make quite a difference in your RPM.
Here's a website with some very useful calcualtors....
http://tri-5.chevyrides.com/speedo.php