Borg Warner OEM T-5s had TWO different speedometer "drive" gears; one with seven teeth (7T) or one with eight teeth (8T).
7T = '83-late '89
8T = late '89-'95
A speedometer "driven" gear with 21 teeth (21T) is correct for 3.73:1
only with a 7T drive gear. Your '93 T-5 does not have a 7T drive gear, it has an 8T drive gear. An 8T drive gear with 3.73:1 requires a 24T driven gear, which isn't made. Closest one is 23T. It isn't a Ford gear, it doesn't have the correct pitch to made with the Ford drive gear, and it will eventually wear out, leave plastic pieces in your transmission fluid, and cause the speedometer needle to bounce or stop moving altogether. Not to mention that since 24T is not 23T, the speedometer is not going to be completely accurate.
You have three choices:
- Use the 23T driven gear and deal with the eventual consequences.
- Change the drive gear to 7T or 6T and use the appropriate driven gear.
- Have a speedometer repair shop install a "gear reducer" in your speedometer cable.