RPM guage/tachometer

i was wondering is their a way to install a PRM guage on a car with out one because i just recently installed taht fan overide swith on my car and when i turn it on the car drops in power. but i need to get ne altornator or get it cleaned. but yeah like a couple months ago i seen oil light come on and i just want to know what the RPM's of the enging is when it does this.