speedometer error

[ALfactor@domain.elided]

For lack of better company, I took my GPS for a ride in my '74 spider to 
check the speedometer.  I was surprised to find it indicating about 12% BELOW 
actual speed.
Every other Alfa I've been associated with ('67 and '74 GTV's) all were 
optimistic by at least 10%. They all have/had stock tires and wheels.

I'm curious as to what the range is, and thought it would make a good rainy 
day thread. Has anyone else checked their Speedo vs. an accurate car, GPS, or 
police radar gun (intentionally or otherwise)?  

Mitch Fisher
Dublin,  CA

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