tach question

[Houston]

The speedo stopped working in my 87 325 eta a couple of years ago. I found a dash in a junk yard out of a 318 and put it in the 325. I can't remember what the car redlined at with the old tach. But, it easily goes over 6,000 with the one out of the 318. I'm thinking it is set up for a 4 cylinder and I am not getting the correct reading. There is no way an eta engine would rev that high, right.
How can I get it to read the rpms correctly?
Thanks.

[MrM3]

i would think it would still read correctly, i once put a eta tach (5k rpm gauge) in my 325is and had no problems with accuracy. warm up the car and try to push it past 4500rpm, if the revlimiter kicks in at an indicated 4500rpm then the tach is accurate, if its way off then just try to find a proper eta tach.