[rohrt]

Can some tell me how many teeth are on the red drive gear?

[HWYSTR455]

17 . Here's the chart including driven gears:


http://www.tciauto.com/tc/speedometer-gear-calculator/

It has a calculator for determining which gear(s) you need there too.

It is possible to go outside the range of what the driven gear can drive, it either won't make contact or will eat the gears quickly.

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[HWYSTR455]

May have quoted the drive gear, 21 teeth on a red driven gear. (I can't see pics that are hosted on other sites).

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[rohrt]

I found this on a chevelle site. So I would guess I'm running the 17 tooth Red?

THM400 Speedometer Drive Gears
Part
Number Teeth Color Clip Price
8629547 15 Brown/Gray None $ 3.61
8623311 16 Orange None $14.34
8629548 17 Red None ***
8629549 18 Blue None ***
8629550 19 White/Yellow None ***
8629551 20 Green/Brown None ***
8623316 21 Black None ***

[HWYSTR455]

The drive gear goes on the output shaft of the trans, the driven gear goes in the bullet and attached to the cable. Not sure if I have that reversed, but believe that's correct.

Drive gears are marked with a stripe, you can look through the bullet hole with a flash light and should be able to see the stripe. Sometimes the whole face of the drive gear is painted.

So a red driven gear is 37 teeth, according to the chart on TCI.

You can use a sharpie, mark one tooth, then go around and count.

I only use programmable speedos now, so it's been a while since I've dealt with this.

.

[HWYSTR455]

Low number of teeth usually points to a drive gear, driven gears have more teeth.

.

[HWYSTR455]

I believe each tooth on a driven gear equates to @ 2.5 mph change.

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[72projectbird]

Good info!

I have to replace my speedo gears. My speedo hasn't worked since the LS swap, and I finally got around to diagnosing the issue. I pulled it apart and the gear in the trans is MIA

[HWYSTR455]

Originally Posted by 72projectbird

Good info!

I have to replace my speedo gears. My speedo hasn't worked since the LS swap, and I finally got around to diagnosing the issue. I pulled it apart and the gear in the trans is MIA

4L60E? It supports electric speedos, so doesn't need a gear. You can get a cable-x box. Or, of course, go to a programmable electric speedo.

Another option is to send your speedo out and have it converted. In the long run, that's the best idea. (If you want to remain factory appearing)

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[rohrt]

I didn't realize how easy this was. I always thought the back tail section of the trans had to come off. I suppose it would if you have to change the drive gear. I see there are several ways to calculate the needed driven gear.

I used the TCI calculator asking for the drive gear teeth, the rear end ratio and the tire diameter and it comes to 44 tooth gear should be really close.

I also found that the driven gear I have now should be about perfect when I switch to 3:08 gears.

http://www.tciauto.com/tc/speedometer-gear-calculator/

Pic before changing out the seals due to leak.

[HWYSTR455]

It used to not be so easy, and there was some homework involved when I went through this exercise. I had a full set of driven gears and would drive by speed trailers to zero in. I just recently gave away all those gears.

Like I mentioned, now all I have to do is hit a few dip switches and wah-lah, it's done. On the CompuShift controller for the 4L80E, you just tell it what tire size etc, and it makes the appropriate adjustments, don't even have to hit the dip switches.

Most of the speedo companies now make VSS bullets for popular transmissions, so if you ever need an OE speedo refurbed, have it converted, saves tons of trouble. New tire size? No problem. New rear gears? No problem.

I will probably look at the pics when I get home, curious now...


EDIT: Oh yeah, one big advantage of going to an electric speedo is, no more speedo cable. If you ever go to a E trans, the speedo cable hole/grommet is the same size used for the trans harness, so no drilling holes in your classic either.

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[pastry_chef]

26 inch tall tire?

3:42 gear?

[rohrt]

Originally Posted by pastry_chef

26 inch tall tire?

3:42 gear?

close. I have 25.5" tires and 3:36 gears.

[pastry_chef]

Originally Posted by rohrt

close. I have 25.5" tires and 3:36 gears.

Cool.

With a 17 drive, 44 driven
My numbers suggest the speedometer will read 2.67 % faster than road speed.

[72projectbird]

Originally Posted by HWYSTR455

4L60E? It supports electric speedos, so doesn't need a gear. You can get a cable-x box. Or, of course, go to a programmable electric speedo.

Another option is to send your speedo out and have it converted. In the long run, that's the best idea. (If you want to remain factory appearing)

.

Nah, its a Th350. Old school trans behind a new school engine

[HWYSTR455]

Originally Posted by 72projectbird

Nah, its a Th350. Old school trans behind a new school engine

Pretty sure you have to drop the tail housing to install the shaft gear.

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[72projectbird]

Originally Posted by HWYSTR455

Pretty sure you have to drop the tail housing to install the shaft gear.

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Yeah, I've just been procrastinating. I really should do it soon though.