Tach Swap - now tach not accurate

[603Racing]

Base info: '90 325i, m20b25

My previous tach did not work (broken plastic gear). I purchased a cluster from an early e30 with an m20, '87 or '88 i believe. Both clusters are VDO. I swapped the working tach out of the early cluster into my cluster. The other cluster had painted needles, so I swapped my needle to the tach I put in my cluster.

So, now the 'new' tach is about 250 rpm's higher at idle. I figured this was because I put the needle on and did not get it in the exact right spot. However, I think it's off by more than 250 at higher RPM's. For example, I spun her way up the gauge and the tach read 7k and it didn't hit the limiter.

[nando]

if the coding plug didn't change, then either the needle is off or the tach isn't any good.

[603Racing]

Other than production date, the coding plugs had all the same production numbers. I'm going to adjust the needle to see if that helps. Are there any suggestions on how to do that accurately?

[nando]

I don't think there's a mark on the face like there is for the speedo, so trial and error unfortunately. unless you have a bench test setup..

[603Racing]

I did a test of my own to see if the gauge has a static difference or a linear difference. It appears that it is a static difference, in that it's off by about 300 rpms at idle and at red line.

Yes, my test was to bounce off the red line in 3rd gear. I found third to be more accurate than first or second due to the rate of movement in the needle in the lower gears.