The Drive gear will depend on your rear axle ratio, size of rear tires, and the number of teeth on the drive gear in the trans. This may help.
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http://www.hanlonmotorsports.
The best place I have found to buy the gears is the Ford dealer. Bought one last week for about $4.00. You will also need the clip and o-ring.
If you need a CR manual I have one for free, just pay the postage.
Wayne