I think must people here treat it like the oil pressure gauge, as long it shows something it's working. My gauges are never constant either, the oil gauge is low until I rev it then it climbs. I don't often watch my volt meter but I bet it does the same thing just not much movement. If you want exact readings I have seen trucks with the after market gauges installed in the original gauge holes or just added else where. If I can't fix my tach someday I might just install an after market gauge.

You can call me Fez
'95 SR 15x8 steel rims w/ 33x12.5 Duratracs, cranked torsion bars, "Baja" lights
'09 Nissan Sentra SR