My error on the manual trans link. Sorry. This TCI link stated that most of the Ford Auto trans (including the C-3) speedometer gears are the same.
http://www.tciauto.com/tc/speedometer-gear-calculator/You need to:
1. Look in the transmission opening and see if you even have a DRIVE gear (on the output shaft). If yes, note the color as it indicates the number of teeth.
2. You still need a calculator to know what DRIVEN gear (on the Speedometer cable) you will need.
You tell the Calculator what the DRIVE gear is, your rear ratio and your tire diameter (which you will have to measure). It will then calculate the number of teeth needed for the DRIVEN gear on the cable.
IF... you have no DRIVE gear then start at the middle sized gear, add in all that I mentioned above and see what DRIVEN gear it states. If there isn't one available in that size then try a different DRIVE gear. There are only so many DRIVE and DRIVEN gear sizes and you will need to figure out what it is you need by trial and error if the DRIVE gear is absent. BTW, if you have no DRIVE gear you will need to remove the driveshaft and rear tail housing to get a DRIVE gear on the output shaft.
On my Studebaker's GM 700R4 I not only had to change the DRIVEN gear, but it's housing too because as the number of teeth increased, the gear needed to shift location. Thankfully on the Ford I don't believe they have that need.