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Old 03-11-2009, 04:34 PM
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Differential Gear Calculations

I should be able to figure this out and it's most likely been hammered to death on this site, but, what is the equation to figure out what gears you need to correct your speedometer? OK, rear differential gears. I have a 65 Impala SS that came factory with a 6/three-on-the-tree. Since then it's been upgraded (?) to a 350/Saginaw 4 speed. Now I'm going to a 396 (correct block #) and a TH400. But I think none of that matters. It has 3:21(?) gears in it now and the speedometer is WAY off.(slow) It has a 10 bolt rear that I'm considering rebuilding with new axles and HD ring and pinion (Richmond) and I would like to know how to figure out what gears to use for correct speedometer readings. This will be for street use only, so I'm pretty sure the 10 bolt will be tough enough.

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Old 03-11-2009, 06:34 PM
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howdy there,

the 10 bolt will be fine unless youre gonna big block it an constantly hard race it with slicks on pavement or something else crazy, or if it already has a problem such as worn bearing or gears, loose pinion nut, etc.

there are lots of speedo gear ratio calculators on the internet and on billions of internet auto forums.,

do a google and you will find pages of them

or jut swap the speedo gears from the 3 speed onto your 400

to check for accuracy take it on the freeway and while driving 60mph exaclty, use a stop watch to time yourself between two concurrent mile posts
if you get to the next post before 60 seconds then youre going faster than 60mph and you need to slow the speedo down by putting in a larger driven gear, or a smaller drive gear

get to the post later than 60 seconds then you need the opposite thing.


hope i dont have that backwards


good luck

Last edited by fast68; 03-11-2009 at 07:11 PM.
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Old 03-11-2009, 07:12 PM
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Check out this site
http://www.transmissioncenter.net/sp...____va.htm#350
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Old 03-15-2009, 11:48 AM
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Still confused

OK, those are great sites, BUT, I have a large 44 tooth driven gear with a 7 tooth driver. Are all gears interchangable as long as they are for the correct transmission? Is it a game of switching grear till you hit it right?
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Old 03-15-2009, 01:51 PM
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Here's a calclulator:

http://www.bgsoflex.com/speedo1.html
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Old 03-15-2009, 10:26 PM
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http://novaresource.org/speedo.htm

http://novaresource.org/speedo2.htm
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Old 04-02-2009, 04:28 PM
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Differential Gear Calculations

WOW Jim. That site really helped. I ordered my new gear and it arrived today. Oh man! it's a steel gear and mine is plastic. I called PACT and they assured me it was correct and that plastic TH400 gears haven't been made in over 30 years. So How do I get this sucker on my output shaft? Should I heat it as I do harmonic balancers or just use the BFH method?
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Old 04-02-2009, 08:25 PM
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If the TH-400 is like the TH-350 which I'm assuming they are, then it should slip right on. Have you taken off the old one? There is a removable clip that is easily removed with a small screwdriver. Once the clip is removed, very light taps with a rubber mallet/screwdriver should back off the old one. Careful not to damage the shaft, slip the new one on in the same method. Again, careful of the shaft and don't tap it in too far, it is possible. Slide your clip in the original location and button it up.
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