Tachometer reading too high

almost done with my 67 Barracuda have it running but i noticed somthing the other day. The reproduction tach rests at zero with the car off but once you turn the key to the on postion it jumps up to 500 even before you start the engine. after you turn it on it read about 500 rpms too high at idle. Haven't tested it at other rpms.

I am running an MSD 6AL 2 box. MSD billet distributor. I don't have a tach adapter not sure if i need one. Just trying to figure out what I can do before i remove the dash. Any thoughts? The tach is from year one.