Ford doesn't make a 23T (speedometer driven) gear for the T5 or AOD transmission. They go as hgh as 21T. There are 23T gears available but the teeth do have the correct pitch to mate well with the speedometer drive gear on the output shaft. The teeth are also thin, since there are so many of them on the gear. So what you have there is a combination for wearing out the gear and circulating plastic bits inside your transmission. Not a very desirable situation.
With a T5 you could change the 8T drive gear to 7T or 6T, as the drive gear is clipped onto the output shaft, but with an AOD you would have to tear the transmision down completely, as the drive gear is machined into the output shaft. And there is only 8T and 7T AOD output shafts available, no 6T.
You could get a speedometer repair shop to install a "gear reducer" in the speedometer cable. That's what stadair is talking about.
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