The 20-tooth black gear is part number C1DZ-17271-A. It would work with 3.73:1 gears if you've got the 7-tooth drive gear which was stock for most T5's.
I've copied the table from the FRPP catalog and have a javascript calculator for the speedo gears on my website. Just follow the URL in my sig.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team