I found this rpm calculator and wanted to post it up. If you are running an A340 or any other tranny with a final drive of .70 to 1 it will work. Using this tool I found a 500 rpm difference between my stock 4.30 gears (2213) and my new 5.29's (2720) at 70 mph with my 33's.
I am swapping in a 4.3 so now I know I should go back to my 4.30's for best results.

http://www.700r4.com/speedoCalc/rpmcalc.shtml

 

Theres some good ones on here too...

left side of the page under "calculators"

http://www.4lo.com/