Speedometer Failure

Hello all my name is:Dave
I have a: (93 300GT VR4) and am located near New Buffalo, MI. Had the cam belts replaced last year by South Suburban Mitsubishi in 2004.

Speedometer now reading very low, but aero flap on trunk lid opens and closes at what seems to be the right speed, so at least part of the speed sensing is working.
Thoughts of others have also seen this problem, or do I just take it back to the dealer. Several at about the same inconvenient distance as are 2 shops that claim foreign car experience, so I’ll call to see who sounds like they know what to do.
Thanks
Dave
It has the following issue: (be sure to describe it as best as you can. Remember to also tell us what you have tried so far)

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