I don't know this car specifically, but there's probably a speedometer cable and a sensor. The problem could be at either end--with the sensor or with the connection at the dash.
I recommend getting a repair manual for this car and checking the diagrams--they probably have a manual at your local library. That'll tell you whether or not you can replace the part yourself--could be a big job.
Your 93 ford taurus has both a speed sensor used for your cruise control and antilock brakes system and it has a speedometer cable that operates the speedometer.If tyhe speedometer is not operating corectly the cable is suspect as is the speedometer head itself.
I,m not sure on the Ford Taurus,but most all cars dropped the cable and use a sensor to relay the transmission out put speed.Most are located on the transaxle assy,on the passenger side of the vehicle or off center in that direction.The sensor has a 2 wire hook up, and inside the housing uses a reluctor coil and a spoked wheel,that makes a signal or pulse, like the solid state ign system for the eng.This goes through the circuit to the computer moduale and then to the speedometer head.