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Just got the vette out for the first ride of the year and the speedo isn't working I have yet to look at it - at this time I'm gathering data as to the design of the speedo circuit and ideas as to what might be wrong.
Isn't the circuit totally mechanical with a cable from the trans to the speedo? I suspect that the cable is the most likely culprit?
I would remove the cable from the transmission and use a drill to run it slowly backward and see if the needle moves. If it does then the speedo and cable are ok so it's either the driven gear or the drive gear in the transmission. Driven gear is pretty easy to replace, drive gear, not so much.
If the needle doesn't move then its either the cable or the speedo that's at fault. I would replace the cable and retest.