Factory Tachometer is Overreading

Hello everybody! I just installed a factory tach into my 67 barracuda which has electronic 'orangebox' ignition. I found the tach on eBay. It works fine, but when I give it even a little gas, it jumps up to 4-5k RPM which is obviously wrong. I did some forum searching and found some threads talking about special sending units and calibration, and I'm looking for a little more information please.

Thank you in advance :)