aftermarket tach?

i installed an aftermarket sunpro tack in my 96 eagle talon esi...i got it all wired up light comes on gets power but when i rev up the rpms are 2k rpms off....what can cause this??

is it autometer cause autometer usually does that...U just wired it to the wrong wire thats all. Its the middle wire that controls the coil pack.

whats wrong with the stock tach?? lol y do you need 2?