Here is a neat tool.
http://www.5speedtransmissions.com/gear_calculator.html
You put in the information you have like RPM and tire size and and the gear ratio in your rear end and it will give you everything else. If you don't know the ratio of the diff, put in 3.55 and see what comes out. You can change any of the input data you need to. It also gives you a way to determine where you will be if you change from? to?