The '51 thru '53 Ford-o-matics had a speedo driven gear made of fibre-cement. I believe they switched over to nylon in '54 and kept this design in '55 & 56.
The gear was available in 19, 20, 21 and 22 teeth, depending upon which rear-end ration you had.
Many cars with Ford-o-matic commonly had 3.30:1 rear end ratio with 670x15 tires and they used the 19 tooth nylon gear. Color coding of the repro gears is identified at the very end of the gear bump-out that rides on an inner boss inside the rear extension housing. Otherwise, the nylon gear is all white.
Along with the color-coding, the number of teeth is identified at the flat end of the gear as well as part #.
In 1957, the aluminum rear extension housing is different than the '54/55/56 and uses a different gear design.
The nylon driven gear with the fewest teeth rotates the mechanical cable core faster than a gear having more teeth and vice-versa. So if your speedometer is showing that your speed is slower than you are really going, you need to swap your gear for one having fewer teeth and vice-versa.
Do not swap a '54/55/56 aluminum rear extension housing with a '57 or later extension housing on full-size cars. The rotation of the driven gear is backwards on the '57 and later extension housings. If you have a '51-53 Ford-o-matic, they have an all cast iron rear extension housing and the repro nylon gears are supposed to work in these, but you may experience a little bump-and-grind of the speedo needle, because the boss inside the cast iron housing doesn't stick out that far to meet the little protrusion on the end of the nylon gear. During those years, Fomoco designed a little clip to hold the fibre-cement gear onto the end of the cable core. The nylon gears don't have that provision. It wasn't necessary due to the re-designed height of the boss.
