Miatas typically read a little higher than actual speed, but not usually that much off. I assume that 66-66 MPH is a typo. Does the reading actually vary as you drive along at the same speed?
I don't know, someone here probably has a better idea of what is causing the problem. You might want to ask at the miata.net forum as well. The answer to your question is very likely in the archives there.
A faulty speedometer cable usually makes noise, and the gears on the one with 120,000 miles looked as good as the one with 75K miles, with no visible damage or wear. Just a guess and a fairly easy one to check without needing to have testing equipment.
You don't happen to have a friend with a 1996 or 1997 model that would let you do a test-swap of the instrument panel, do you?
My best guess is that the problem is at your speedometer/instrument panel, but like I said, electronics are something that I usually leave to the experts. I was mainly commenting on the speedometer being mechanical.
Pat