200 RPM discrepancy between analog tach and E.I. Inc digital tach in my M20J

[Bill_Pyles]

Which instrument is likely to be giving a false reading and why?

[201Mooniac]

Given the technology I would guess the EI more likely to be correct but it is best to test them with an optical tach to verify which, if either, is reading correctly.

[A64Pilot]

Any chance the tach could be set for a 6 cyl or something?

Whichever one doesn’t read 2700 on takeoff is the incorrect one is my bet.