I saw a YouTube video where a guy looked on the Cope Racing Transmissions site and somehow accessed a section that described how to interpret the 10,000 day calendar for part numbers.
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The 9589 is a number that fits between January 1987 and January 1988, making this a genuine 1988 model transmission. There were no 2.45 gear 904 based transmissions built this late so this is indeed a low gear version.
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I got an email back from this guy:
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It is the early version, the 3 wire. I emailed back my phone number. Hopefully he calls. I'd use this for the Dart before I'd use the 998. This would allow me an overdrive, putting a 3.21 or 3.23 gear down to around 2.21 !
It might make sense to use it in the black Dart....
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That car has a 4.10 diff, 2.89 in overdrive. I don't drive that car for distance though.