Speedometer Calibration - Page 2 - Ford Mustang Forum
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Originally Posted by mcc351 View Post
My speedometer is clearly not accurate and is now worse since I changed to a different rear axle ratio. I was able to find formulas on-line as to how to calculate (also a website that calculates it for you) the correct number of teeth for the driven gear. My question is how do I know what drive gear I have? Do I need to take the <acronym title="transmission"><acronym title="transmission"><acronym title="transmission">trans</acronym></acronym></acronym> apart to get to it? Or is it a standard thing? This is on a 69 Mustang with an automatic FMX mated to a 351w. It originally had a 3:25 rear that is now a 3:50 rear. Does anyone have a similar assembly?


This should help you correct the issue

* Common Drive Gear Teeth:

C4, C6, or FMX: 8 Teeth



http://www.classic-cougar.com/tools/...speedocalc.php



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post #17 of 19 (permalink) Old 06-08-2019 Thread Starter
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The issue I have now is that I need a 22 tooth driven gear but can’t seem to find anyone that sells it. The ford parts catalog for mustangs only goes up to 21 tooth. I probably need to replace the drive gear to make this work... but not sure how to remove the drive gear easily.

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As has already been mentioned, you can't change the drive gear. It is machined into the output shaft of Ford automatic transmissions, including the FMX you have.
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Quote:
Originally Posted by mcc351 View Post
The issue I have now is that I need a 22 tooth driven gear but canít seem to find anyone that sells it. The ford parts catalog for mustangs only goes up to 21 tooth. I probably need to replace the drive gear to make this work... but not sure how to remove the drive gear easily.
Using a 21 tooth driven gear will get you pretty close.

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