#1  
Old 09-26-2020, 01:40 PM
Tony_D Tony_D is offline
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Default Need help determining speedo gear 200-4R replacing Super Turbine 300

I have a 200-4R transmission out of a 1985 Monte Carlo SS that I want to put into my 1966 GTO convertible. The car is a 389-4bbl with Super Turbine 300 trans.
It has 3:23 rear end gear ratio where the Monte Carlo came with 3:73 so I believe I need to replace the speedometer gears in the transmission so the speedometer indicates correctly.

I found a chart that determines what speedo gear ratio you need (drive gear/driven gear) based on tire size and rear end ratio. Using 26 inch tire and 3:23 it says I need a 0.399 ratio. The closest on the chart is 0.400 which corresponds to 12 tooth drive gear and 30 tooth driven gear, GM PN 8634935 and 561035 resp.

Can someone verify these part numbers and my thought process?

Thanks

  #2  
Old 09-28-2020, 03:31 AM
lust4speed's Avatar
lust4speed lust4speed is offline
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Your math is correct. Would be interesting to pull the speedo drive out of the trans and look in the hole and see what color the drive gear is. It would be less work if you can just make the driven gear swap. Don't know if you can get lucky and find the red 12-tooth drive gear already in place. There's a possibility of matching a different drive gear, but not a lot of choices available on driven gears. You can run the calculator for different sized drive gears and see if you get a close enough match (although the 12/30 is really close). I like the TCI site to do the quick and dirty calculating for me: TCI Speedo Gear Calculator

I use Amazon or eBay and the gears are quite a bit cheaper than going through a trans company's website.

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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
  #3  
Old 09-28-2020, 08:18 AM
Tony_D Tony_D is offline
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That's a good idea; I'll check the drive gear when I get a chance. I did a search on gear prices and was surprised how expensive they are/can be ($30 for a plastic gear!).

Thanks for your feedback!

  #4  
Old 09-28-2020, 08:41 AM
Chief of the 60's Chief of the 60's is offline
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Swapping out the speedo gear on a GM tranny is a 5 minute job. Not on the 200. Get it right the first time and good luck.

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