[GMCnet] TH-425 / TH-400 Speedo Drive Gears

[Rob Mueller]

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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[Kerry Pinkerton]

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, near Huntsville,

77 Eleganza II, "The Lady", 403CI, also a 76 Eleganza being re-bodied as an Art Deco car hauler

[Don Adams]

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

[Craig Lechowicz]

Don,
In my old dealership days, we changed driven gears without changing the drive gears. As I recall, 1 tooth of change was worth somewhere around 3 mph at 60 mph.
--
Craig Lechowicz
'77 Kingsley, Waterford, MI

[RC Jordan]

yeah, i've had driven gears changed by 1 or, maybe, 2 teeth back in the 70's without changing the drive gear.
--
77 Royale Coachmen Side Dry Bath "http://RetroChicGMC.com/"
76 Birchaven Coachmen Side Wet Bath "http://ShortCircuitGMC.com/"
Both currently at GMCcoop

[Ken Burton]

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

[Don Adams]

Great, thanks guys!

So now Craig, if the speedo is 3 mph low I want it to spin faster to correct. Is that 1 tooth more or less ??
My head is apinning to decide which :)

Thanks again

--
Don Adams Dallas, TX
'76 26' Glenbrook, '90 Sidekick
rebuilt by R Archer, powered by J Bounds, Koba

[Dan Gregg]

When I was messing with mine, I got the various gears from Jim K. Never could get it just right so I quit. GPS does not lie.
Dan
--
Dan & Teri Gregg
Dexter, Mo.

http://danandteri.blogspot.com/

[Kerry Pinkerton]

Don A wrote on Sat, 29 December 2012 21:56
> ...Is that 1 tooth more or less ??...


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, near Huntsville,

77 Eleganza II, "The Lady", 403CI, also a 76 Eleganza being re-bodied as an Art Deco car hauler

[Matt Colie]

WD0AFQ wrote on Sat, 29 December 2012 23:08

> When I was messing with mine, I got the various gears from Jim K. Never could get it just right so I quit. GPS does not lie.
> Dan

Another nice thing about using the GPS as a speedometer....
When we go to Canada, it can become KPH very easily.
Matt
--
Matt & Mary Colie
'73 Glacier 23 Chaumière (say show-me-air) Just about as stock as you will find
SE Michigan - Twixt A2 and Detroit

[Craig Lechowicz]

Don,
What Kerry said. 1 tooth less on the driven gear should get it "around" 3 mph faster.

--
Craig Lechowicz
'77 Kingsley, Waterford, MI

[Kerry Pinkerton]

Here is my box of drive gears. I had a neighbor who was moving and he gave me a bunch of 'stuff' like this. I never throw anything away and my kids are going to have to deal with it all someday. :twisted: 8o 8)

http://www.gmcmhphotos.com/photos/data/6256/D_015.jpg



http://www.gmcmhphotos.com/photos/data/6256/D_016.jpg

--
Kerry Pinkerton

North Alabama, near Huntsville,

77 Eleganza II, "The Lady", 403CI, also a 76 Eleganza being re-bodied as an Art Deco car hauler

[Chris S.]

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? Thanks!
--
Chris S. -
77 Kingsley mostly OEM -
S.E. Michigan

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[Sean Kidd]

You want the style G speedo gear...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

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[Sean Kidd]

Having difficulty adding images lately from iPad

--
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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[Chris S.]

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

[Ray Erspamer]

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

--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC...@gmail.com
414-484-9431
Web Site: http://ray-lisa.page.tl/

[Ray Erspamer]

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.

[Sean Kidd]

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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[Ray Erspamer]

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.

--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC...@gmail.com
414-484-9431
Web Site: http://ray-lisa.page.tl/

[Emery Stora]

Ray

I got an adapter with my 3.55 final drive from Jim several years ago. Due to tire diameters mine was off a couple of percent. I returned it to Jim and he had the gears in the adapter changed and then it was right on target.

He doesn’t just change the adapter but he provides ones with different gears inside to take care of his various final drive ratios and the tires that his customers use.

Emery Stora

[Chris S.]

I thought I had found a speedo/driven gear that would work, but it was not the correct size gear to replace the one we have. I'm back looking at
external reducers. I believe I found somebody that can build custom reducers which are based on kits from Stewart Warner, based on the chart found
here:

http://www.garbee.net/~cabell/december/datcon/~hpa0002.jpg

It sounds like they still make parts for these to get an exact ratio. I have talked to them and they are very knowledgeable on these. They said to
drive and use the odometer (not the speedometer) to try and calculate miles shown on the odometer vs. actual miles traveled. Either with mile
markers, or in our case using Google maps to calculate miles between two points. As he explained, the odometer should be accurate but the speedometer
may vary (which we have found to be true) depending on the speed and also due to age of the instrument and other factors.

I will try them out and if it works out, will post back more info on our findings.

--
Chris S. -
77 Kingsley mostly OEM -
S.E. Michigan

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[Gary Kosier]

Chris,

I've got almost all the gears for the Stewart-Warner gearbox. I bought the
inventory of an old speedometer shop that closed years ago.
The problem is getting the box. I tried to bribe the guys in the junkyard
to save them for me, but they didn't want to bother with it.
If you can find the box, I can probably re-gear it for you.

Gary Kosier
77PB w/500Cad
Newark, Ohio

--------------------------------------------------
From: "Chris S." <t...@apex-internet.com>
Sent: Thursday, October 01, 2015 12:39 PM
To: <gmc...@list.gmcnet.org>
Subject: Re: [GMCnet] TH-425 / TH-400 Speedo Drive Gears

[Chris S.]

Kosier wrote on Thu, 01 October 2015 16:57

> Chris,
>
> I've got almost all the gears for the Stewart-Warner gearbox. I bought the
> inventory of an old speedometer shop that closed years ago.
> The problem is getting the box. I tried to bribe the guys in the junkyard
> to save them for me, but they didn't want to bother with it.
> If you can find the box, I can probably re-gear it for you.
>
> Gary Kosier
> 77PB w/500Cad
> Newark, Ohio

Gary,
Thanks for the offer! I ended up purchasing a box before I saw your note, but I don't have a box to use anyway (returned it). I already received the
new one and will be testing it out this weekend hopefully. If all is well I'll post back the results and the info for the group, as it seems these
are getting very hard to find. Good thing you have the parts!

[Chris S.]

Thanks everybody for the feedback. I ended up hooking up the reducer we purchased and it's working well. The person I spoke with was very
knowledgeable and it sounded like they have a fair amount of experience with these. I mentioned we were looking for a reducer for the Toronado TH425
and he said the reducer is the Stewart Warner 800 series. He said they still make some of the parts and they are able to make quite a few different
ratios still. The turnaround time was one day for them to build a custom reducer and get it out the door. Cost is $125 + ~$8 for shipping.

In our case, we ended up needing a .8500 ratio reducer for use with the 3.70 FD and 75R16 tires. Ratio is calculated by the distance shown on the
odometer, divided by the real miles travelled (obtained by a map, Google, GPS, etc.). I made about a 25 mile trip and checked, and it's right on,
(give or take 1/10th of a mile).

The place we purchased the reducer is Texas Industrial Electric. Reducers are on their website:
http://www.texasindustrialelectric.com/800_Series_Speedometer_Adapter.asp . They replied via email and I called and talked to them to confirm the
ratio and place the order, very easy to work with.

--
Chris S. -
77 Kingsley mostly OEM -
S.E. Michigan

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