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Old 10-19-2007, 10:15 AM
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whats my gear ratio?????

o.k. quick question...
how do I figger out my rear end gear ratio ?
Is there some kind of mathematical formula??
It's a 79 Chevy C20 with 8 lug axels and a 400TH w/ the bolt on yoke .
I know none of those have anything to do with my gear ratio , just giving some info....
Tires are 245/75/16 . ( about 29.5 " tall )

Thanks in advance for any replies

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Old 10-19-2007, 10:34 AM
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Divide the number of teeth in the ring gear by the number of teeth in the pinion gear.
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Old 10-19-2007, 11:35 AM
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If you can get an Accurate speed at an Accurate RPM, you can use any of several calculators available to get an idea of the ratio.
JA
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Old 10-19-2007, 11:38 AM
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Quote:
Originally Posted by johnnya
If you can get an Accurate speed at an Accurate RPM, you can use any of several calculators available to get an idea of the ratio.
JA
what or where are these magical calulators that you speak of ??
Is there a formula : like tire size x speed divided by Pi R squared????
JK about the pi thing .... Pies R round - not square
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Old 10-19-2007, 01:06 PM
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whats my gear ratio?????

4:10 ratio
jack one side up mark tire at bottom ,
mark drive shaft ,use a jack stand as reference point of mark on side of shaft .
turn the wheel by hand ,and count the number turns on tire mark and the number of turns on shaft .

turn tire 4 turns and see if shaft turns 10 or eleven turns to the tire going 4 turns

or get grease'y and do the devide thing above .
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Old 10-19-2007, 01:07 PM
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i think you can put the rear on stands, turn the driveshaft one turn and see how many times the tire turns. but you might have to devide by two or multiply by two depending on the posi or not factor. maybe somebody else can help you out with this variation. i cant remember. but i think the only way other that searching the tag on the rear, which should say what the ratio is, is to pull the cover and do as in emkaz's reply.
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Old 10-19-2007, 01:16 PM
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whats my gear ratio?????

times ten or you would have 4 turns shaft to one turn tire =4 to one
multiply it and you got 4.10

pull the cover ,
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Old 10-19-2007, 01:43 PM
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Easiest way is to mark the tire & driveshaft. Then jack up only one rear wheel and go TWO full turns of the tire, exactly, while counting the turns on the driveshaft.

Those gm gears are 456, 410/411, 373, 321 So, if the shaft turns a tick over 4 turns but Not 4-1/2, then you have a 410/411.

Or about 3-3/4 turns would be the 373.


If it is a posi, jack up both wheels and go ONE full turn of the tires.
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Old 10-19-2007, 01:46 PM
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Quote:
Originally Posted by F&J
Easiest way is to mark the tire & driveshaft. Then jack up only one rear wheel and go TWO full turns of the tire, exactly, while counting the turns on the driveshaft.

Those gm gears are 456, 410/411, 373, 321 So, if the shaft turns a tick over 4 turns but Not 4-1/2, then you have a 410/411.

Or about 3-3/4 turns would be the 373.


If it is a posi, jack up both wheels and go ONE full turn of the tires.
got it !
Thanks to all who responded !
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Old 10-19-2007, 02:16 PM
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Yeah the tire and driveshaft method is great for scrounging in junkyards for those "good" rear axles
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