Syncing Tachometers

[Bob S]

I have a 42' aft cabin with twin cummins diesels and have always had a problem with my tachometers. I am determined to try to get at this problem this year. I have 4 tachs, two for each engine because I have 2 helm stations, main and bridge. At higher RPM"S none of them seem to agree between the main read and the bridge read. I have synced my cruise speed by sound mostly. I have a portable test tach. Any suggestions on troubleshooting? Any wiring diagrams out there? Or should I just get a pro to look at it? Thanks