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I have 3.70 gears in my Galaxie's 9" rear end with 27.8" tall tires.
I've determined my C-6 trans has a 9 tooth drive gear. I've used the appropriate formula as well as 4 different calculator sites and get the same answer... a 24 tooth driven gear, which I cannot find. Ford has them from 16 to 21 teeth. Jegs has a 23 tooth gear for sale.
Can anyone tell me approximately how far off the speedometer will be if I use the 23 tooth. Example...if my speedo says 65, about how fast am I really going??? I quess I will just have to learn to live with it if I can know how to figure the approximate difference.
Thanks :)
 

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One tooth on the driven gear is worth about 3 mph.
Agreed. Also, since you need more teeth, the speedo will read on the fast side vs ground speed. 60mph on the speedo will equate to approximately 57mph ground speed if you are one tooth shy.

I normally compare the speed that the car shows to that given on a GPS as there may be some other variant in play.
 
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