Yamaha Engine Meter

[52Alumacraft]

I have a Yamaha Eng-Meter-4C-02 to record engine hours. It also reads RPM when the engine is running but I have never looked at it while the engine is running because it is under the cowl. Yesterday while running with the muffs on I noticed that it was reading about 3000 rpm while the tach on the dash was reading about 2000 rpm. It sounded more like 2000 than 3000.

Has anyone had a similar problem with this unit? I was thinking it may be for a 6 cylinder engine (1.5 ratio is about right) but I don't think they make one of these for a 6 cyl.
JB