Your 91 TBI auto should have 2.73 gears from the factory. That would mean that you need a Electric Speedo sensor with a 37-38-39 tooth gear on it. ( Probably a Red 37 tooth Driven gear if you had 15" tires from the factory )
The
EARLY type electric speedo sending units required a specific speedo sensor for certain tranny driven gears. See this pic for an example:
What you will need to verify is that the speed sensor you buy is correct for a 90-92 tranny (so it matches the existing tranny plug ) and is OK for a 37-39 tooth gear. I do not recall if the 90-92 style sending unit is as specific as the earlier style - meaning the 90-92 sending unit might be a "1 size fits all" sending unit.
I don't have a Sending unit like your "handy" so I can't look it over to see if it specifies a gear range on it - but perhaps another member has one they can look over for Ya' in advance of your purchase ! ( or you could pull yours out and look it over !?)
Hope this helps !