how do i find the exact rmp my car is running b/c my stock tack at idle reads 1200 and my aftermarket tach reads 800 rmps so i was just wondering which tack is right. also i have a cam in the motor so i think the comp is confused and i think it does idle higher that normal idle.
thanks.

 

First off, it is RPM, Revolutions Per Minute

Second, it is a TACH short for TACHOMETER

What kind of tach do you have, is it wired correctly? What motor are you running? Is the stock tach for a v8 car? Give some more info.