Mike,
To mis-quote Arthur Conan-Doyle, "when you've eliminated all but one cause,
whatever is left must be the suspect"
The only suspect left in your case is the speedo drive pinion, which is
removable, in-situ. I'm pretty sure this is nylon, so if the gear is
stripped, there should be no collateral damage to the gearbox. A new pinion
should have your speedo running again.
As to the speed at 3,500RPM, if by stock you mean without O/D, then the SI
does 17.3MPH in top gear per 1,000RPM. That gives you 60.55 MPH at 3,500.
The above is as per the factory manual, and assumes you have the
non-overdrive 3.89:1 differential.
Having said that, you should not rely totally on the rev-counter reading, as
they are not always as accurate as they might be. Non-standard tyres will
also make a difference.
Cheers,
Julian.
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