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Discussion Starter #1
I finally have time to work on my Bird. I swapped over to 3:55 with trac lok. I want to correct my speedo and from what i'm reading here, i need the orange 20 tooth, is this correct? I have 255/60-15 tires on rear and they were very close to correct with stock rearend gears. TIA
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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You need a 19 tooth, not a 20.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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The method to calculate needed speedo gear is to multiply the number of output shaft teeth (7) by the rear gear ratio (3.55) and the number of tire revs per mile (745), then divide by 1000. Your result is 18.51 because your tire is so large. As the tire wears down and decreases in diameter your speedo will get more accurate.
 

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Discussion Starter #5
Is there a 19 tooth available for my transmission? Only see a 19 for 8 tooth output shaft. Mine is a 7 tooth shaft.
 

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Is there a 19 tooth available for my transmission? Only see a 19 for 8 tooth output shaft. Mine is a 7 tooth shaft.
Same gears, but the number of teeth you need for a certain rear ratio changes (by 8:7 or 7:8, depending on which way you're talking about).

PATC shows a 19 tooth gear at http://www.transmissioncenter.org/Ford_Speedometer_Gears.htm .

They're local to me, but I can't say past that. That, and they've been there 90 forevers, it seems *grins*

Before you order, what was your old rear ratio, what gear did you have, and what's the new ratio?

Ideally, you compute the number of teeth you need from the same ratio as the one between the two rear ratios, and order that.

That is, if you went from 3.08 to 3.31, and your old one was an 18 tooth, then you'd want ( 3.31/3.08 ) * 18 teeth on it ... or in your case, 19 teeth (nearest match).

RwP
 

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Discussion Starter #9
Ordered one today from local Ford dealer, be in tomorrow $7.95 plus tax. :)
 

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