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Old 07-25-2012, 08:04 PM
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T-350 speedo gear question?

Ok, so to start off here's what I've got: an 83 Grand Prix with a TH350, 3.42 gears, and 235/60/15 tires. I'm curious what gears I'm supposed to put in the trans to get the speedometer close? I found several places on-line that list the gears..but I'm a little confused on the driven/speedo gear part.

Here's what I came up with: 17 for the trans gear, and 45 for the speedo gear. BUT, which ones are considered the drive/driven gear?

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Old 07-25-2012, 08:40 PM
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Check out this site man. ALL you need to know.
Know your tire diameter , rear end ratio and away yiu go.
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4
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Old 07-25-2012, 10:33 PM
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Quote:
Originally Posted by tobyj17 View Post
BUT, which ones are considered the drive/driven gear?
The drive gear is mounted on the tranny output shaft. The driven gear is the one that's easily removed that is mated to the speedometer cable. The driven gear always has the greater number of teeth of the two gears. Hope that helps some.
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Old 07-26-2012, 12:03 AM
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The drive gear is the gear that drives the driven gear. The driven gear is the gear that is driven by the drive gear.
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Old 07-26-2012, 03:03 AM
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Ok, I think I got it. Thanks!
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Old 07-26-2012, 11:45 PM
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Originally Posted by cobalt327 View Post
The drive The driven gear always has the greater number of teeth of the two gears. Hope that helps some.
That is true for TH350s, but is not always true for other applications. Example, I know that some Toyotas use a 27 tooth drive and a 22 tooth driven as well as other similar combinations. Same with Subaru with 23 tooth drive and 19 tooth driven.
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Old 07-26-2012, 11:47 PM
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Talking in this thread about a TH350. But thanks just the same.
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Old 07-27-2012, 08:23 PM
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I haven't ordered the gears yet, but i did finally get the speedo hooked up, 30 mph off!

One thing i forgot to ask, are these gears the same with a TH350 and Th 350c?
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Old 07-28-2012, 01:06 AM
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Quote:
Originally Posted by tobyj17 View Post
I haven't ordered the gears yet, but i did finally get the speedo hooked up, 30 mph off!

One thing i forgot to ask, are these gears the same with a TH350 and Th 350c?
Yes and no. The TH350C uses a driven gear that has less teeth than the TH350 may have. The TH350C is usually 17 to 25 teeth or so and the non C has more in some cases. The size of the housing will be larger for the larger (30-35 tooth- plus) driven gears, IIRC. You should be able to narrow this down doing a search on it.

Last edited by cobalt327; 07-28-2012 at 01:16 AM.
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Old 07-28-2012, 01:50 AM
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Too late to edit the last post, so I will try to clarify by saying that there are two size speedo gear housings that fit into two different tailshaft (extension) housings. Because the tailshaft housings will interchange, either tranny may now have either housing on it.

I don't think the gears pitch will allow a small housing driven gear to work w/a large drive gear or vise versa, if it did the speedo error would be huge.

The speedo gears will be in sets that have considerably different counts to them. You should be able to select the right size of gears by the driven gear housing size you have.

I hope this doesn't confuse the issue worse.
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Old 07-29-2012, 07:48 PM
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Quote:
Originally Posted by cobalt327 View Post
Too late to edit the last post, so I will try to clarify by saying that there are two size speedo gear housings that fit into two different tailshaft (extension) housings. Because the tailshaft housings will interchange, either tranny may now have either housing on it.

I don't think the gears pitch will allow a small housing driven gear to work w/a large drive gear or vise versa, if it did the speedo error would be huge.

The speedo gears will be in sets that have considerably different counts to them. You should be able to select the right size of gears by the driven gear housing size you have.

I hope this doesn't confuse the issue worse.

Basically it sounds like I just need to pull it apart and see what I've got.. I didn't see any different gears that identified themselves as being for a TH350 or 350C.. I'll have to do some more digging.

Thanks!
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Old 07-29-2012, 11:49 PM
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Quote:
Originally Posted by tobyj17 View Post
Basically it sounds like I just need to pull it apart and see what I've got.. I didn't see any different gears that identified themselves as being for a TH350 or 350C.. I'll have to do some more digging.

Thanks!
In most all cases, you aren't going to find the wrong drive gear used w/the wrong tailshaft housing- but the possibility exists. It's prolly easier to keep the housing you have and match the correct drive gear to it if it's wrong. But like I say- that's going to be a rare occurrence. A glance will tell you if you have a large or small speedo housing:


Large housing


Small housing (upside down)
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Old 07-30-2012, 08:22 PM
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I've got the large housing... so which gears does a person order?

Last edited by tobyj17; 07-30-2012 at 08:27 PM.
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Old 07-31-2012, 12:46 AM
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Quote:
Originally Posted by tobyj17 View Post
I've got the large housing... so which gears does a person order?
I would contact Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4 or a similar company w/the knowledge to tell you what ratio of speedo gears to use along w/whether you need to use a different speedo gear housing (not talking about the whole tailshaft housing, just the part that holds the driven speedo gear). There's a calculator you can use to figure it out what gears you need, then check to see if the housing you have is compatible w/the needed gear.

Example of the gears available are also shown at http://www.areds.com/wp-content/uplo...rs-no-tags.pdf
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Old 07-31-2012, 07:04 AM
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I just got an email from TCI that included info on speedo gears- possibly because of posting about them here at Hotrodders: Speedometer Gears - Transmission Accessories - TCIŽ Auto
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