I think what '91 is talking about is the 7 or 8 tooth speedo drive that is machined into the output shaft of the AOD. Unlike the T5's, the drive gear of the speedo is integral with the output shaft; the only way to change the drive gear is to replace the entire output shaft...not a pretty picture.
Anyways, the easiest way I can say to do it is to remove the speedo driven gear from the transmission housing then use a flashlight and mirror to look down into the speedo hole and count the teeth.
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Jeff Chambers
T/S #3
11.97 Seconds/114.5 MPH
Chambers Racing Team