Chevrolet Repair: 91 Lumina Euro Speedometer/Odometer, speed sensor, transmission speed

Question
My grandson was given a 91 Lumina Euro that had no transmission.  His uncle had a used transmission installed (year unknown).  The speedometer/odometer now read about half what they should.  Odometer read 2 miles after traveling 3.5 miles.  He has been told that the transmission speed sensor drive gear has to be replaced ($700) to match the gear to his car.  Is there any oter option?  Can the speedsensor scale factor be changed in the ECM?  

Thanks--Jim

Answer
Hi Jim, I have never heard of this before, I would replace the speed sensor on the transmission because the one in there might be from the used trans and made for a different type of car. There is no gear inside the trans to change for this problem.

Let me know how you make out

Mark