So I found this formula in NPD's catalog on how to calculate how many teeth you need for the driven gear on your tranny. It goes like this:

(# of drive gear teeth) x (Axle ratio) x (Tire revolutions per mile)
----------------------------------------------------------------
1000

"Just plug in the number of teeth on the drive gear inside
the transmission (6, 7, 8) multiplied by the axle ratio
(3.00, 3.50, etc.) and multiplied the tire rev’s per mile,
then divide all that by 1000. The result, such as 19.27 is
the approximate number of driven gear teeth required for
proper speedo readings. Tire revolutions per mile can be
found in the manufacturers specifications for a given tire
style and size."

I need to know how many teeth on the drive gear inside my transmission I have. How do I find out?
I checked my tire manufacturers website and found the tire revolutions per mile. My front and rear tires are different sizes. Do I go with the rear tire size or the front tire size? Or should I find the average of the two?

 

Also, is the gear in the picture below a RH or LH gear according to NPD's catalog pic (below)? I think it's a RH but according to NPD's catalog and an article in Mustang Monthly, C4's should have a LH gear.

 

You have their type 3 gear. Use the wheels that the speedo watches. The others will have no affect.

Use the current numbers for your speedo to calculate the number of teeth in your tranny gear.

 

So the speedo should be watching the rear wheels right? What do you mean by " Use the current numbers for your speedo to calculate the number of teeth in your tranny gear."

 

You know what rear end you have now and your speedo is pretty much accurate with the current driven gear (or it's accurate for the original rear end). If you have/had 3.00:1 gears with stock height tires and a 20 tooth driven gear, you can calculate relatively quickly what the drive gear is. Then, use that number to solve the equation for the new tires and rear end ratio.

Shoulda stayed awake in algebra class.[sm=bustedsign.gif]

 

The gear in the trany is ether 6 or 7 teeth 7 is for 2.70 to 3.50 rear gears and 6 is for 3.50 to 4.30 gears

 

I think there is an unwritten law somewhere stating you don't have to think over break. Thanks for the info guys

 

I need to place a speedo gear on my cable the person I bought the car from put on a new cable with a gear. I have a 66 coupe 289 C4 automatic tranny with a 9" rear but am not sure of the ratio. I guess I'll use the guidance this thread has presented. Any guesses on speed driven gear teeth and type 3 or 3a???

 

Any guesses on the question above??? Cable without a gear??

 

I'd bet a 9" would have 3.50 to 4.30 gears. So that means your tranny would have 6 gear teeth. If you have a C4 then you will need the same gear I have but with a different # of teeth. Type 3 and RH. I don't know how to tell what type of gears you have though, but I bet you're going to have to take the center section out and look at them.