There are several rpm/speed calculators online.
Input a few Rear ratios, Tire sizes etc.. to see what the rpms will be.
Here's a few:
2nd down >> http://hotrodworks.net/hotrodmath/quarter.html
I wouldn't go highwer than 3.73 if you plan on crusing the road. 3.42, 3.55 are good all around ratios.