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Old 05-26-2008, 12:18 PM
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??? ON speedo gears

HI,just wanted to know what color or how many teeth count driven gear is needed for th400 with 27'' tall tires with 3.73 gears.and need to know on the pressed on gear to that slides on the end of tranny shaft. thanks.
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Old 05-26-2008, 02:58 PM
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Do you happen to know what tooth drive gear it has in it now?
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Old 05-26-2008, 05:35 PM
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its a tci competition tranny not one installed on end.
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Old 05-26-2008, 07:35 PM
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Quote:
Originally Posted by 1972bigblockchev
HI,just wanted to know what color or how many teeth count driven gear is needed for th400 with 27'' tall tires with 3.73 gears.and need to know on the pressed on gear to that slides on the end of tranny shaft. thanks.
It is simple math really ...

1) A mile is 5280 feet and there is 12 inches in a mile ... so there is 63,360 inches in a mile.

2) A 27 inch tall tire is 84.82 inch around ( like measuring your self for a belt )
... 27 inches X Pie ( 27X 3.1417 ) = 84.82 So a 84.82 inch tire would turn 746 times in mile. ( 84.82X 746 )

3) With a 3.73 gear ... the driveshaft will turn 2786 times in a mile ( 746 X 3.73 )

4) The speedometer needs to turn 1000 times in a mile ( to read correctly ) and the driveshaft is turning 2786 times so the ratio of the gears needs to be 2.786 to 1 ... for the cable to turn 1000 times.

This is a photo of a 700R4 speedometer set up but the 400 Turbo uses the same principal set up.



A 15 tooth metal gear on the output shaft and a 42 tooth gear on the " bullet " end will give you the correct ratio The 42 tooth gear is color coded GREEN ...



You need a different " bullet " for the 40 to 45 gear ... the other bullet is used for the 34 to 39 gears.

Since you have a TCI transmission ... you may want to contact them for the speedometer drive parts ... their parts page is here ... http://www.tciauto.com/Products/comp...eter_gears.asp
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Old 05-26-2008, 07:54 PM
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Seeings how the colors indeed differ in color here is some GM part numbers for what your looking for. Drive gear 15 tooth>(PN:8629547) Driven Gear 42 tooth(PN:1362049) Driven Gear sleeve "housing"(PN:1362293)
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Old 05-26-2008, 09:52 PM
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http://novaresource.org/speedo.htm has a formula and charts with potential gear combinations for you.
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Old 05-27-2008, 03:29 PM
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thanks guys ill give the chart a try.
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