^ that is good advice.

Another idea is to have a voltage comparator checking the alternators output, my friend created something very much like what you're describing with that as the input. Higher RPM's => higher voltage, but this is coming second hand.

 

You could probobly have the needle of the tachometer hit small switches as it gets to the desired rpms..Like have the power wire run along the bottom of the needle then have contacts for each different LED where the needle would hit the rpms and the lights. That would be a much easier and kinda ghetto way of rigging it. Forgive me if you don't get what I'm saying, I'm tired.

 

sounds kinda neat. id like to see some pics.