95 buick speedo

My wife just bought a 95 Buick Riviera 3.8 supercharged/automatic overdrive, the trans was rebuilt/replaced by the GM dealer $2700.00, long befor we bought it, previous owner very car illiterate,
everything works fine, but, the speedo registers 15 mph slower than the car is moving, I sort of dought Mr. Good wrench would have not put the correct drive gear in the tranny for the speedo, and am assuming this is an electric drive for the speedo, could this be a bad drive unit, or is it out of calibration.... any ideas welcome......I told the wife parts for this car would be spendy, so guess who gets to buy the parts, and fix it...Mark
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operation.
Rey
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