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Either Honda BF75 tachometer or nmea 2000 reported RPM is not accurate

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WalleyeBrian

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I have a 2019 Honda VF 75 and was wondering if my prop is optimal for the boat, so I started checking speed and RPM relationships, then I noticed that the RPMs reported on my tachometer differ slightly from the RPMs reported on my Lawrence and or Garmin units via the nmea 2000 connection (see photos for example). They only differ by a few hundred RPMs.

Any idea why they don’t match, and which one if either I should believe?

Thanks!

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I asked Bard (Google’s large language AI model) why my Garmin and tachometer are reporting different. RPMs and in part of it’s answer, it said “
  • The tachometer and Garmin unit are using different reference points. The tachometer is typically measuring the RPM of the crankshaft, while the Garmin unit is typically measuring the RPM of the propeller. This difference in reference points can lead to a small difference in RPM readings.
Could Bard be right?

I’d like to get a new prop that gets me higher RPMs and a faster boat, but I don’t want to damage my outboard by over-revving it. Greatly appreciate any help!
 
Wrong, the nmea reads the rpm from the ecu and will be the correct rpm. The honda analogue gauges are crap and inaccurate
 
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