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Discussion Starter #1
I posted a while back on my driven gear for the speedometer. 68 Std, C4. At an actual speed of 60 mph, my speedometer reads 90 mph. My driven gear has 19 teeth. The math would suggest I need a gear with 28 1/2 teeth. Sounds like something an engineer would specify with the extra half tooth. Common sense would dictate either a 28 or 29 tooth gear, however the local Ford dealership says that Ford only goes up to 21 teeth for driven gears. Is he a knuckle head parts counter guy, or is he right and I'm just generally screwed??

Ron.
 

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In the Mustangs Unlimited catalog it's 16-21 teeth, so no I don't think he's jerking you around.

Their gear formulas is:

Driven Gear Teeth= (Drive gear Teeth x Axle Ratio x Tire Rev Per Mile)/ 1000

Their Note: Select driven gear with the closest whole number of teeth. There is 5280 feet in a mile.

Tim B
 

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Discussion Starter #4
I'm running a 4:10 posi rear, 9". Is the shaft that the worm "drive" gear is on replaceable? It seems to me that 16 to 21 teeth on the driven gear isn't a broad enough range to accommodate the wide range of rear end ratios, so the drive gear must be a variable too. Kind of a pain to change, and I suppose they're expensive.

Ron
 

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Ron, I dont claim to be an expert on this but I think your on the right track. For what its worth I just changed one on my truck last year and it was only like 20-25 bucks from Ford if I remember right. mm
 

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Speedo gearing

Yes, there are three available drive gears for the output shaft (you will have to remove the tailshaft housing). They come in 6,7 and 8 teeth. With the different driven gears (that's the one on the end of the speedo cable) ranging from 16 to 22 teeth, you'll have a whole multitude of available gear ratios.

Another option is to call your local tranny shop and see if they have an intermediate reduction box. You'll need a two-piece speedo cable then.
 
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