www.richmondgear.com
They have a calculator there that tells you what gear you should run for a desired rpm. IE - you tell it you want to turn 3000 rpm @ 70 mph and put in tire diameter and it tells you what gear to run.
I've used it a few times to figure out which gears to run at different dirt tracks for my stock car and it seems to be fairly accurate.
Hope this helps!
-Dana