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Old 04-01-2005, 11:43 PM
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TH 400 Speedo Gears

Well, here is one that I hope someone knows. Have a TH 400, 1967 vintage, that I just got from a friend running it in a 68 Camaro with 396. Car was set up for strip, trans is full manual, reverse pattern, billet shafts, all internals upgraded with the heat treated sprags, etc. At a c note, I wasn't going to hesitate. I'm looking at putting it in my 67 RS/SS with a 454 ( 450HP) ( rear end yet to be decided on )that will see both street and strip, but 75 percent will be the street. Now comes the problem. The speedo housing and gear, which on this short tail model is in the end of the trans case and not the tail housing, has a myriad of available gears...Being as this was used on the strip, a plug had been used in the inch and 3/4 hole where the speedo drive gear and housing was inserted. I can get the new housing and o rings from the local GMPP shop, but am mystified as to what gear to get. There are two housings, one for gears with 20-32 teeth and another for the 33 and up. These speedo drive gears are color coded ( green has 4 different shades for 4 different numbers of teeth ). I am assuming that these teeth are somewhat related to rear end gearing but I have no clue where to start. Has anyone torn down their TH 400 lately and if so, what color gear do you have? And I am assuming that these will vary with year of trans, gear ratio, etc. So if you could include those as well.....Just looking for a starting point so I can at lease know which casing to get as a start...... Thanks....Dan

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Old 04-02-2005, 06:06 AM
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DO NOTHING........

Until you get the rear end. Then tire size and gear ratio will tell you which ones to get........

here is a site with all the information you need.once you have the rear end ratio and tire size.


"I won't be wronged . I won't be insulted, and I won't be laid a hand on. "

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Old 04-02-2005, 09:12 PM
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Thanked 1 Time in 1 Post is another good speedo site.
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Old 04-02-2005, 11:19 PM
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You won't be able to accurately determine what speedometer hears you need until you get the rear gear ratio you want, AND the tire size you plan on using.

Both of these are the deciding factors in determining what speedometer gears you need to use to get the gear reduction necessary to give the speedometer itsself the correct input ratio.

I have heard an awful lot about a speedometer input of 1000 rotations per mile, but on the other hand I have heard of a few other per mile rotation count speedometers. I have also heard (but never checked for accuracy) that there is an input value (rotations per mile) on the back of some (not sure here either) if not all speedometers.

I have quite a few speedometers I salvaged from many parts vehicles I have dismantled in the past, and have to work in the shop later on today (I am on here at 1:19am) so I will try to check.
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Old 04-03-2005, 10:30 PM
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Here's yet another nice link that explains most of the same information as the ones mentioned above, as well as the "part number" for each different gear. Hope this bit of info helps in some way.
Speedo Gear Selection
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Old 04-03-2005, 11:58 PM
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if you are just itching to do something RIGHT NOW, buy one right in the middle.
Then, get one of the inline adapters, so you can tailor the adjustment later. If you have cruise control, get one that will fit inline just ahead of the cruise unit, so you don't have to crawl under the car, to change it. Another plus to that is being able to easily change the speedo reading, if you decide to change tire size, or axle ratio.
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