speedo gear question

[19stang66]

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?

 

[19stang66]

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.

 

[dennis112]

A C4 uses gears like the one on the right (marked RH) so you and the pic are correct. The one marked LH is for a Toploader. Why they are both 3A's I don't know.

They are not interchangeable so be sure that you get the correct orientation. A quick call to NPD should clear things up.

Some 70's model C4's used the one at the top of the pic, but they are rare.