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For the bunch of us rebuilding our differentials and changing ratios, finding new speedo gears for the trans can a problem. Using the first two websites here, it was easy to figure out and get the right gear.
The 'Chevelleengineer" site explains how to figure it, and then lists the gears available.
The "Kiesler" site has a speed calculator you can use to figure the actual speed difference for your setup instead of using road testing.
The "Redrider" site I just now found, haven't studied it yet, but it looks to have a lot of info about parts for various applications.
Once I had the right gear chosen, I called the Chevy dealer and gave him the part number. He had it for me that evening. Only bitch was they only come in two-packs, so now I have a spare 'blue 38t driven' gear for a 3.36 behind a TH400. They're cheap, it was still worth buying two to get one.

Later, John


http://chevelleengineer.home.comcast.net/speedo.htm

http://www.keislerauto.com/tools/gm_analyzer_5sp.html

http://www.redrider.us/Speedometer_Gears.htm
 

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John I need one for my 72 that I just changed from 308 to 336's. If you want to sell it I'll take it. What color was the 308 gear? I think mine is an off white? I have not removed it yet to get a tooth count.

Gary
 

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John I need one for my 72 that I just changed from 308 to 336's. If you want to sell it I'll take it. What color was the 308 gear? I think mine is an off white? I have not removed it yet to get a tooth count.

Gary
Here's the speedo gear chart.



 

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Thanks Duke
I remember you posting this before. The TH 400 gear is different then the 4 spd gear. Going from 308 - 336 is close and may only off 5 mph or less.

Gary
 

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Thanks Duke
I remember you posting this before. The TH 400 gear is different then the 4 spd gear. Going from 308 - 336 is close and may only off 5 mph or less.

Gary
I have the TH400 chart too. Do you need it or are you all set?
 

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If you want to post it for the others that would be good.
 

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I have to know in my own mind, the speed of the cable is 1000 rpm for indicating 60 mph....allways WAS so, as I recall....

but for the life of me....my speedo seems right for 60-70 mph....

then again, I"m indicating 98-100 mph, which dont seem like it, wife agrees, and we barely keeping up with traffic, so it seems....

been paced by wife's car and a buddies truck.....

so....IS IT POSSIBLE SOMEHOW, that a speedo can be NON LINEAR in i'ts response???

:crazy:
 

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I know for a fact that my speedo is ~10% high. I dont care that much that its wrong, but am unaware of what causes this. Obviously the correct gear would fix it, but where in the drive line is the gear located? Is it in the diff as some of you have alluded to? If so how big of a project is it to pull out and see what I have?
 

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I know for a fact that my speedo is ~10% high. I dont care that much that its wrong, but am unaware of what causes this. Obviously the correct gear would fix it, but where in the drive line is the gear located? Is it in the diff as some of you have alluded to? If so how big of a project is it to pull out and see what I have?
It's in the tailshaft of the tranny. The driven gear is easy, the drive gear necessitates removal, at least in autos.
 

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It's in the tailshaft of the tranny. The driven gear is easy, the drive gear necessitates removal, at least in autos.
Thanks. I learn something new every day.

:cheers:
 

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Resurrecting an old thread...

...proof that the search function works!

I have a question that I'm hoping someone can help with.

The Muncie M21 in my '72 LT-1 seems to have eaten the driven gear. The old one is natural with 19 teeth. when I look at the chart on the chevelleengineer site I see 2 natural 19-tooth gears. PN 3987919 and 3860342 which are compatible with drive gears of different outside diameters.

The problem is that I have no clue what the OD of the drive gear is. The trans is in the car and I don't plan to remove it. Anyone know which gear I need?

Thanks,
Scott.
 
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