Speedometer Gears at the Transmission


reliable source of info
I've been searching for quite awhile for a simple way to select the correct driven gear for the TH400 in my '58 truck, but have not found anything.
If you have something already posted on this forum, please give me a hint on where to find it. Searches did not turn up anything. Lots of other stuff on rear gear selection, TC stall selection, stuff like that, but I just want to put in a gear that will be as accurate as I can. The speedo hole is plugged right now, and I need to get the speedo hooked up. Have a new GM cable the right length, and the empty gear holder, which looks like this one below, and is marked with the numbers 36, 37, 38, 39; but mine is also marked 2 LD:


reliable source of info
Thanks Grumpy,
Here's one of those links that worked well for me
for my cable driven (mechanical) speedometer...







With an input of :
1000 revolutions per mile
- (speedometer factor, which can vary depending on the speedometer)
30.5" rear tire diameter
- (this is for my pro street '58 pickup)
3.50 differential gear ratio
15 number of transmission speedometer drive teeth;

-The link calculated that a driven gear required
35 teeth to be accurate.

After using the above link and calculating this driven gear spec,
I found this 2nd link that shows the various drive gears and driven gears
available (scroll down a ways) for a GM TH400 transmission:


Which also has an on site calculator which gave me the same
numbers, and confirms that my drive gear of 15 teeth is gray color;
and my driven gear of 35 teeth is pink....

And the data shown on this link:


confirms that my 15 tooth drive gear (gray) will work with my 35 tooth (pink) driven gear...

Many Thanks Grumpy....
The internet is an amazing place...
'2Loose' Willy

this might help


The next step is to remove the driveshaft. Do this by disconnecting it from the rear end. You will need to remove "4" 8mm bolts and then you can pull it out gently from the transmission. Before you remove it though, be sure to mark the driveshaft and rear end with a line for easier installation. Also, you imight want to tape up or use rubberbands to keep the "spiders" from falling out of the spicer, its a pain to reassemble if you do drop them, so be careful.


Now the driveshaft is out.

Next lets remove the driven gear. (the grey gear in the picture below)

In the picture below you can see the small piece attached to the tail shaft housing that we need to remove. One bolt holds on a retaining clip which you need to remove. After that, you can just wiggle out the piece that holds the driven gear in place. Watch out though, a little tranny fluid will spill out.

This is what we removed. The green gear is what we are changing out, it just slides off.

Now, looking through the hole that the driven gear just came out of, we can see the red drive gear that we need to replace next.

To get to the drive gear, we need to remove the tailshaft housing which is held on by 4 bolts. In order to get to the top passenger side bolt, the C-beam needs to be moved. Remove the two bolts that hold on the C-beam up front and only one bolt that holds it on in the back. This will let you slide it to the side without completely removing it, as you can see to the right of the picture below.

Remove the red drive gear by lightly tapping it with a hammer and chisel to shatter it, just be sure not to lose the clip that holds it on, because you will need it to attach the new gear.

Next, slide on the new drive gear and secure it with the clip. Then thats it! All you need to do now is install everything, which is opposite of how you took it out. Just be sure to lube up the tailshaft and driven gear housing O-rings with tranny fluid before you re-install.

To help find which gear you will need to use with your set-up, refer to this speedo gear calculator. **LINK** Courtesy of member CentralCoaster

Total cost to replace driven and drive gears = $5.50

Now go out there and enjoy your vette!


http://corvetteforum.net/c4/vettenuts/S ... shtm.shtml





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The Grumpy Grease Monkey mechanical engineer.
Staff member
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