'87 Spider Speedo calibration?

["Dean W. Cains" <dwc@domain.elided>]

Hi Group:

On the rear of the instrument panel, obviously behind the speedo, there are 
three small holes.  Two look like openings for a test probe, but the third, 
located at about the 5 o'clock position as viewed from the rear, looks like 
the adjustment potentiometer I've heard a little about.  True?

Which way do I need to turn it to increase the reading of the 
speedometer?  To be clear, the speedo now reads ~50 km/h when I'm driving 
at about ~100 km/h.  Otherwise, it works OK, and the odometer seems to be 
very accurate.

Thanks in advance.  
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