I have a 1986 El Camino with a OEM techometer at reads 2.5k when engine is at idle and 6k at a speed of 50mph. What do I do to fix the problem

It sounds like the tachometer has a bad ground. Where did you get the tachometer? Was it a factory install or did you install it?

Better than mine, that I just installed. It picks any random RPM and stays there until I shut of the car. Although this car doesn't have the stock coil inside the distributor cap so I'm sure it is due to that! Good luck with yours.