I actually know this one!!!!
Zmanco, like the 75-78 the 260z's tach is voltage triggered. There is a resistor in the line running off of the negative side of the coil. The resistor is housed in a fuseable link case in the passenger side footwell.
Racer88, the resistor is a 2.2k ohm resistor! just got finished replacing mine for one out of a 280zx.