You do need a 23 tooth gear, but you dont need any reducer. I have the gear, and ordered it online from Anderson Ford. The gear is an aftermarket part, as Ford does not make any think bigger than 21 tooth. The quality isnt quite as good, but it works. Also, they only make one gear for the T-5 and the AOD, even thought the factory AOD gear is slightly different than the T-5 one (longer tail piece on the gear). I called anderson about this, and was told there is only the one gear available, but it would work in an AOD. Mine has worked fine so far, but hasnt been installed for very long. The word on the street is that it might last forever in an AOD car, or it might get chewed up in a couple months, just depends on the individual car.
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1993 GT/AOD
'93 Mustang GT