Spider Speedo Advice Sought

["Rob Favali" <favali@domain.elided>]

Friends,

The speedometer on my 1993 Spider Veloce is off on the average of 15 MPH
above 30 MPH.

The interesting part is that the odometer is correct based on multiple
measurement checks and confirmations (measured miles, side-by-side car
highway travel measurements, etc.) Plus the tach is correct (I have learned
to drive by the tach which I have come to prefer.) Using a RPM-to-MPH
conversion and confirming it while having a friend drive along side me with
his car, it appears to be working correctly across multiple RPM-MPH checks.
The car has 195/60-15 phone dial wheels on Pirelli P6000 tires.

However, I would like to get the speedo working correctly.
I have installed a new cable but have not addressed the sending unit or the
gauge itself.

Any ideas?
Is there a test a can do to the sending unit to confirm its correct
operation?
Am I faced with a unit removal and calibration?
Any help would be appreciated. My suspicion is that since the odometer is
correct it must be in the gauge itself.

TIA,
Rob Favali
Burlington VT
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