Home » Public Forums » GMCnet » [GMCnet] TH-425 / TH-400 Speedo Drive Gears
| [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194282] |
Fri, 28 December 2012 19:55  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
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G'day,
This subject was being discussed under the Ready for Tires thread and I thought it would be a good idea to start a new thread so we
can reference it later:
Kerry,
You may have ASS U ME'd correctly! I just did a Google search and came up with this:
http://www.jegs.com/p/TCI/TCI-GM-Speedometer-Gears/762271/10002/-1
The first group of gears with the smaller gear look like the speedo gear that is used in the TH-425.
Regards,
Rob M.
-----Original Message-----
From: Kerry Pinkerton
Again, I don't know about the 425 but the Turbo 350 & Turbo 400 trannys have replaceable gears. I have a whole box of them all
color coded by the number of teeth. I'm under the impression that the internals of the 425 is pretty much a Turbo400 so I ASS U MEd
one could change the gears. Don't know for sure.
Regardless, the off road guys have converter boxes that allow them to fit all sorts of weird tire sizes and still have the speedo
work.
--
Kerry
Regards,
Rob M.
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194289 is a reply to message #194282] |
Fri, 28 December 2012 21:42  |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
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Rob, I clicked on the details of a couple and T425 is one of the applications so that appears to be doable. That said, since tires aren't available it doesn't really get us much to be able to change the speedo gears unless you change the differential ratios.
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194312 is a reply to message #194289] |
Sat, 29 December 2012 09:02 |
Don A
Messages: 895
Registered: October 2008
Location: Dallas, TX
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Does anyone "know", if you change the driven gear from the normal 19 tooth to 18 or 20, would you have to change the drive gear also?
It seems like an easy fix.
Although if you just need to replace the normal 19 tooth, some local Chevy ot GMC dealers will get it for about half that price.
Don Adams Dallas, TX
'76 26' Glenbrook, '90 Sidekick
rebuilt by R Archer, powered by J Bounds, Koba
[IMG]http://www.gmcmhphotos.com/photos/data/6109/G2.jpg[/IMG]
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194332 is a reply to message #194327] |
Sat, 29 December 2012 13:16 |
rcjordan
Messages: 1913
Registered: October 2012
Location: Elizabeth City, North Car...
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yeah, i've had driven gears changed by 1 or, maybe, 2 teeth back in the 70's without changing the drive gear.
SOLD 77 Royale Coachmen Side Dry Bath
76 Birchaven Coachmen Side Wet Bath
76 Eleganza
Elizabeth City, NC
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194349 is a reply to message #194332] |
Sat, 29 December 2012 20:08 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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Same here we always changed only the plastic gear to correct the speedometer when we changed rear end ratios.
It was very common to do this in the 1960's and the plastic gears were readily available.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #194357 is a reply to message #194355] |
Sat, 29 December 2012 22:42 |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
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| Don A wrote on Sat, 29 December 2012 21:56 |
...Is that 1 tooth more or less ??...
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You want it to spin faster/more so you'd decrease the number of teeth on the DRIVEN gear. That way a given number of rotations of the DRIVE gear would spin the DRIVEN gear more.
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287698 is a reply to message #194282] |
Mon, 21 September 2015 12:39 |
lw8000
Messages: 201
Registered: July 2012
Location: S.E. Michigan
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We've been looking at one of these gears as our speedometer is about 10 MPH fast at 60 MPH real speed (speedo shows 70). This has been caused by the 3.70 final drive and new 16" rims/tires. Based on what you guys said about 1 tooth per 3 MPH, is it safe to add 3 teeth? I see the site you mentioned has a 34-tooth gear for the TH425, which might be just what we'd need as our current gear has 31 teeth.
http://www.jegs.com/i/TCI/890/880007/10002/-1?parentProductId=762271
Is adding 3 teeth a pretty safe change, without changing the driven gear (we're assuming it's the OEM one)? Thanks!
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan
[Updated on: Mon, 21 September 2015 12:41] Report message to a moderator |
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287702 is a reply to message #194282] |
Mon, 21 September 2015 13:52 |
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SeanKidd
Messages: 747
Registered: June 2012
Location: Northern Neck Virginia
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You want the style G speedo gear for earlier years, not sure when they changed...I had to replace mine last month since it was slightly grooved at the seal lip, our speedo gear is the same as the 2004R transmissions from the same years....the TH400 gear will not fit the TH425. Applied has them in stock, takes 5 minutes to change, be sure to get the bullet (gear housing) O-ring too...unscrew your speedo cable, 7/16 bolt holds the clip, bullet slides out. If you have the reduction gear as I do for my 3.70:1 FD, don't drop the little Dickie out of the male end, it's not permanently attached.
Spring clip holds in seal, not a snap ring, just be careful it doesn't fly off somewhere when you pull it out.
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.
Colonial Travelers
[Updated on: Mon, 21 September 2015 13:53] Report message to a moderator |
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287703 is a reply to message #194282] |
Mon, 21 September 2015 14:03 |
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SeanKidd
Messages: 747
Registered: June 2012
Location: Northern Neck Virginia
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Having difficulty adding images lately from iPad... Here is my original 31tooth speedo gear (driven)
Note grove in end from rubbing on seal for 84,000 miles, started leaking...
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.
Colonial Travelers
[Updated on: Mon, 21 September 2015 14:06] Report message to a moderator |
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287747 is a reply to message #287702] |
Tue, 22 September 2015 07:52 |
lw8000
Messages: 201
Registered: July 2012
Location: S.E. Michigan
Karma: 1
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SeanKidd wrote on Mon, 21 September 2015 13:52You want the style G speedo gear for earlier years, not sure when they changed...I had to replace mine last month since it was slightly grooved at the seal lip, our speedo gear is the same as the 2004R transmissions from the same years....the TH400 gear will not fit the TH425. Applied has them in stock, takes 5 minutes to change, be sure to get the bullet (gear housing) O-ring too...unscrew your speedo cable, 7/16 bolt holds the clip, bullet slides out. If you have the reduction gear as I do for my 3.70:1 FD, don't drop the little Dickie out of the male end, it's not permanently attached.
Spring clip holds in seal, not a snap ring, just be careful it doesn't fly off somewhere when you pull it out.
Thanks Sean. Since you also have the 3.70 FD, do you know which gear you went with (number of teeth) and/or reducer ratio, to get your speedo correct? We're in the same boat. We originally tried a reducer that was shipped with the 3.70 FD, however it was not correct and ended up being faulty and locked up completely on us and caused the governor gear to strip. So I've been looking at just swapping the speedo gear and if it corrects it or gets it close we may call it good with that and not even mess with an external reducer on the speedo cable. But I was curious what you ended up using. Thanks!!!
Chris S. -
77 Kingsley, 3.70 FD, mostly OEM -
S.E. Michigan
[Updated on: Tue, 22 September 2015 09:47] Report message to a moderator |
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287751 is a reply to message #287747] |
Tue, 22 September 2015 09:12 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Good Morning
I have a 78 Royale with the 403 and a 3:70 final drive and I agree
that the speedo adapter that came with the final drive is incorrect.
I've played with the gearing on the adapter and I have my speedo dead
accurate to a GPS at 70 MPH.
For the adapter, this is what worked for me:
Starting at the transmission input to the adapter, a 13 tooth gear
turning a 19 tooth, next to the 19 tooth is a 16 tooth turning a 13
tooth. This ratio reduces the speed of the speedometer cable by
.9726.
There are people on the GMC net that have access to gears for the adapters.
Hope this helps.
Ray
On Tue, Sep 22, 2015 at 7:52 AM, Chris S. wrote:
> SeanKidd wrote on Mon, 21 September 2015 13:52
>> You want the style G speedo gear for earlier years, not sure when they changed...I had to replace mine last month since it was slightly grooved at
>> the seal lip, our speedo gear is the same as the 2004R transmissions from the same years....the TH400 gear will not fit the TH425. Applied has them
>> in stock, takes 5 minutes to change, be sure to get the bullet (gear housing) O-ring too...unscrew your speedo cable, 7/16 bolt holds the clip,
>> bullet slides out. If you have the reduction gear as I do for my 3.70:1 FD, don't drop the little Dickie out of the male end, it's not permanently
>> attached.
>>
>> Spring clip holds in seal, not a snap ring, just be careful it doesn't fly off somewhere when you pull it out.
>
>
> Thanks Sean. Since you also have the 3.70 FD, do you know which gear you went with to get your speedo correct? We're in the same boat. We
> originally tried a reducer that was shipped with the 3.70 FD, however it was not correct and ended up being faulty and locked up completely on us and
> caused the governor gear to strip. So I've been looking at just swapping the speedo gear and if it corrects it or gets it close we may call it good
> with that and not even mess with an external reducer on the speedo cable. But I was curious what you ended up using. Thanks!!!
> --
> Chris S. -
> 77 Kingsley mostly OEM -
> S.E. Michigan
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMCRoyale@gmail.com
414-484-9431
Web Site: http://ray-lisa.page.tl/
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287752 is a reply to message #287751] |
Tue, 22 September 2015 09:25 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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I forgot to mention in my last post that I am running larger tires
which makes a difference, I'm running 245 75 16 which have a larger
diameter of 30.6 inches.
On Tue, Sep 22, 2015 at 9:12 AM, Ray Erspamer wrote:
> Good Morning
>
> I have a 78 Royale with the 403 and a 3:70 final drive and I agree
> that the speedo adapter that came with the final drive is incorrect.
>
> I've played with the gearing on the adapter and I have my speedo dead
> accurate to a GPS at 70 MPH.
>
> For the adapter, this is what worked for me:
>
> Starting at the transmission input to the adapter, a 13 tooth gear
> turning a 19 tooth, next to the 19 tooth is a 16 tooth turning a 13
> tooth. This ratio reduces the speed of the speedometer cable by
> .9726.
>
> There are people on the GMC net that have access to gears for the adapters.
>
> Hope this helps.
>
> Ray
>
> On Tue, Sep 22, 2015 at 7:52 AM, Chris S. wrote:
>> SeanKidd wrote on Mon, 21 September 2015 13:52
>>> You want the style G speedo gear for earlier years, not sure when they changed...I had to replace mine last month since it was slightly grooved at
>>> the seal lip, our speedo gear is the same as the 2004R transmissions from the same years....the TH400 gear will not fit the TH425. Applied has them
>>> in stock, takes 5 minutes to change, be sure to get the bullet (gear housing) O-ring too...unscrew your speedo cable, 7/16 bolt holds the clip,
>>> bullet slides out. If you have the reduction gear as I do for my 3.70:1 FD, don't drop the little Dickie out of the male end, it's not permanently
>>> attached.
>>>
>>> Spring clip holds in seal, not a snap ring, just be careful it doesn't fly off somewhere when you pull it out.
>>
>>
>> Thanks Sean. Since you also have the 3.70 FD, do you know which gear you went with to get your speedo correct? We're in the same boat. We
>> originally tried a reducer that was shipped with the 3.70 FD, however it was not correct and ended up being faulty and locked up completely on us and
>> caused the governor gear to strip. So I've been looking at just swapping the speedo gear and if it corrects it or gets it close we may call it good
>> with that and not even mess with an external reducer on the speedo cable. But I was curious what you ended up using. Thanks!!!
>> --
>> Chris S. -
>> 77 Kingsley mostly OEM -
>> S.E. Michigan
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
>
> --
> Ray Erspamer
> 78 Royale - "The Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMCRoyale@gmail.com
> 414-484-9431
> Web Site: http://ray-lisa.page.tl/
--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMCRoyale@gmail.com
414-484-9431
Web Site: http://ray-lisa.page.tl/
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287758 is a reply to message #194282] |
Tue, 22 September 2015 14:40 |
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SeanKidd
Messages: 747
Registered: June 2012
Location: Northern Neck Virginia
Karma: 4
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Senior Member |
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Ray, I'm running the adapter, from JimK, it's spot on compared to GPS...im not sure how many teeth you can jump before the drive gear needs to be replaced. I've only heard one or two with confidence, hopefully one of the resident experts chimes in.
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.
Colonial Travelers
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| Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears [message #287761 is a reply to message #287758] |
Tue, 22 September 2015 15:51 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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I'd love to know what gears are in the JimK adapter because the one I
got with my final drive way way off. He must have changed it.
On Tue, Sep 22, 2015 at 2:40 PM, Sean Kidd wrote:
> Ray, I'm running the adapter, from JimK, it's spot on compared to GPS...im not sure how many teeth you can jump before the drive gear needs to be
> replaced. I've only heard one or two with confidence, hopefully one of the resident experts chimes in.
> --
> Sean and Stephanie
> 73 Ex-CanyonLands 26' #317 "Oliver"
> Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.
>
> Colonial Travelers
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMCRoyale@gmail.com
414-484-9431
Web Site: http://ray-lisa.page.tl/
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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