T-400 Speedometer gear changes - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-08-2010, 09:11 PM
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T-400 Speedometer gear changes

I need to pull the speedometer housing out of my TH400 so I can check to see what gears and what size housing it has. It came from a truck with 3.73s and 28.88" tall tires. I have put it in another truck and it has 4.10s with 28.70" tall tires.

Is it going to leak transmission fluid when i take that housing out? Is it going to pull right out when I take the bolt off the retainer or what?

How do I identify the large speedo housing from the small housing? According to my research, I need a 15 tooth drive gear with a 43 tooth driven gear for my setup, and it looks like i *probably* have a 15/39 or 15/40 gear setup again based on research, but coming from a truck with 3.73s it looks like its possible that it could have 17/38 or 17/39 speedo gears too. I need to know if i have a 15 tooth drive gear already or if i need to order it, as well as the 43 tooth gear. If it has a 15 tooth drive gear i wont have to tear the tailhousing apart which would be awesome.

I suspect that I have the 15 tooth drive gear, but probably have the small gear housing so I will need a large gear housing and the 43 tooth driven gear.

But again i need to pull that apart and verify.

Will it be a certain color or will i have to count the teeth? Do i put the trans in neutral, jack up a tire and spin the driveshaft while counting the teeth on the gears?
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Old 08-08-2010, 11:07 PM
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I always have the nose of the car way down and the rear up fairly high. The front wheels on the ground and GOOD jack stands under the rear end or the drive on ramps ... slid up under the rear wheels ( jack the vehicle up and slide the ramps under the wheels and lower the vehicle ) DO NOT try to drive up backwards on the ramps

Then the fluid is at the front of the case and you can remove the bullet ... and COUNT the number of teeth on the output shaft or check the color. The 40/45 housing is fairly unusual to find but they are out there. My Chevrolet dealer has both in stock ( or did a year ago ).

The gears are color coded by GM

I have found the 15 tooth to be either grey or brown
The 17 is usually red.

The driven gears are also color coded.

37 teeth - red
38 teeth - blue
39 teeth - brown
40 teeth - black
41 teeth - yellow
42 teeth - green
43 teeth - purple
44 teeth - dark gray
45 teeth - light blue

So by determining the color of the driven gear



You can actually determine which housing bullet you have by reading the end of the gear ( color ) where the speedometer cable goes.



Some of the bullets even have the range size that they are ... in a small cast number on the face of the bullet.

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Old 08-09-2010, 11:45 AM
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Thats alot of good info, thank you. The transmission is in a 79 pickup. It had a 6 cylinder originally so now that it has a 454 in it the nose is sagging a bit so that will help get the fluid to the front. I can also jack up the back but it needs to be on stands because I think the wheels will have to turn (thus turning the transmission) so that I can count all the teeth on the gear (unless I can identify it by color which it sounds like I should be able to).

There are a several numbers on the bullet (which i am assuming is what I was calling the speedo housing) in the upper 30s so I am assuming its the small one. That means that i'll need to get a big one so I can use the 43 tooth driven gear.

I'll try to get it apart today so I know 100% for sure everything that I need to know.

Thanks again!
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