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I'm thinking about rebuilding a mark VIII's pumpkin with 3.55 gears. I think that all marks came out of the factory with true lock:leftright I don't want 3.73 as I do drive on the highway every day. Right now I have 3.08 open rear end so 3.55 should make me happy for now. But dose anyone no what gear I would need for the speedometer?
 

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Not sure if this is what you're looking for, but it may be a start. This is from the TECH section of TCCoA.

Which speedo gear to put in?

The 21 tooth is purple, part #D0OZ-17271-B, for an automatic trans. I think you need the 21 tooth gear that is not sold by Ford, but many Mustang shops sell it. Be warned, the teeth are small and the gear wears out fast. Look in the FAQ section of the SCCoA site (www.sccoa.com) for more info on speedo gears and speedometer calibration units.
If your AODE trans has an 8 tooth drive gear, then the above calculations are correct. But if it has a 7 tooth drive gear, then the calculations would be different. If you pull the stock gear first and count the teeth or determine the color and you know your stock rear gear ratio, you can use the SCCoA gear calculator to determine what drive gear you actually have. Then you will know which driven gear to buy. The drive gear is internal to the trans and not easily changed. The driven gear is the one on the end of the speed sensor and is the one that is usually replaced when rear gears are changed.

Car Teeth Color Part #

Auto 18 Grey C7SZ-17271-B
Auto 19 Tan C7VY-17271-A
Auto 20 Orange C8SZ-17271-B
Auto 21 Purple D0OZ-17271-B
 

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All Mark's came with open rears because they had traction controll. All cars came with teh 3.07 gear except for LSC's which came with 3.27's. I wouldn't bother swaping gears unless you are getting trac-loc too.

-Miller
 
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