speedo gear-in tranny
 

Hi All,

Is there different speedo gears for different FOM's? I have the one from the original tranny (orange) and the there is one already in the replacement that is going in (don't know about that one as I have not taken it out yet).

Mike

Re: speedo gear-in tranny
 

The rear end gearing ratio will dictate which speedo gear is used in the tranny.

Re: speedo gear-in tranny
 

Fomoco speedo gears are color coded so you can match your rear gears to your speedo. I believe each color is different by 1 gear tooth. If your speedo is slow compared to your actual speed you would step down( I believe, or visa versa!)) gear teeth to increase your speedo speed. I used to go to a junkyard that had some fords close to my year and grab a few different colored plastic gears.

Re: speedo gear-in tranny
 

Ok, I will use the original one in the new tranny then. Thank you.

Mike

Re: speedo gear-in tranny
 

The 55-56 transmissions use the same type gear, gear drive, and cable. The 57 transmission uses the new style gear and cable. If you go to ant Tbird supplier's website, you will see the different styles. Regardless of year/style, yes the gears are color-coded. It seems that most speedometers are off by 10-15%. My 55 Tbird/w FOM and original 3.31 gears showed 65 mph at approximately 55. The 205/75R15 tires contributed to the inaccuracy, having a smaller diameter than the original 670/15 tires.

Re: speedo gear-in tranny
 

Prior to 1948, the speedometer gear charts also took tire sizes into account. After 1948, the sizes were confined to the rear axle ratio. 17322 is the base part number but the prefix makes the year range that it was used in. The change to the 9-inch rear axle may have had an affect of sizes after 1957 but a lot of the gears are the same or similar. Mercury cars had the Dana 41 and 44 rear axles so they had some different ratios than what the Ford car axles had. The earlier manuals had fiber gears and the automatics changed over to nylon gears. If the color is for tooth count difference then you need to know if you need more or less teeth on the gear and what color represents that tooth count.

Re: speedo gear-in tranny
 

Or get something like this:

https://www.texasindustrialelectric.com/speedo.asp

Or you can use a speedo app on your smart phone.

Re: speedo gear-in tranny
 

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.

Re: speedo gear-in tranny
 

Re-reading the op's post again; as long as you are not changing your original rear-end gears to a different ratio or changing to a different tire size, just count the number of teeth on your original speedo driven gear and select a new gear with matching number of teeth in the transmission you are installing.
If you are swapping to a '57 or later FM, you should be able to remove the rear extension housing and speedo drive gear off your new transmission and install your old speedo drive gear and extension housing and keep your original nylon speedo driven cable and gear.

Re: speedo gear-in tranny
 

If you are using a 57 or later trans, there are many different driven gears (the gears on the end of the cable) (tooth count). A good source for gears is ebay. I have gotten a number of them through ebay.


If you have a pre 57 trans, there is a limited amount available and they are usually available from the old parts vendors.

Re: speedo gear-in tranny
 

You decided not to have the take-out overhauled? You have an exchange trans? If both are of the same year, you would simply swap the one in the present trans into the replacement.

What year is the replacement trans out of?

Re: speedo gear-in tranny
 

DON'T remove the tail shaft to replace the speedometer driven gear! As Paul said, just follow the speedometer cable to the transmission. 55-56 Tbird transmissions use a 90-degree drive that is attached to the transmission where the speedometer cable is attached. 57 Tbirds don't have the 90-degree drive, use a different style drive gear (already said), and the cable has a metal tab that bolts to the transmission to secure the cable.

Re: speedo gear-in tranny
 

The donor came out of a 57 fairlane but it may be a 58 if I look at the number right. And it has the same bolt up so I will plug in the original and hope it bolts up. Now that I think about this and after getting the granny back it is a ‘Doh!’ Post. Thank you all though for putting up with me.

Mike

Re: speedo gear-in tranny
 

Is the 58 trans a COM or FOM? 1958 went to a one piece convertor/different flex-plate (unobtanium). Make sure of what you have.

Do you see any TAG I.D. NOS on either?

The gear style should be the same.

Hey, I don't mind the questions as I learn (or remember) stuff ... :D

Re: speedo gear-in tranny
 

Here is the trans that is going in. Ain’t she purdy! I drained the original-red fluid came out, I mean it was fresh trans fluid. I even took the bolts out of one side of the cross member and started tapping it backwards. So, as soon as I get the rear brakes done the tranny can come out. I did not shoot the flex plate but I have the one that came with it.

Mike

Re: speedo gear-in tranny
 

Before you install your '57 small case FOM, remove the rear extension housing from the '57 trans and remove your old '55 rear extension housing and look at the speedo drive gear on the tail-shaft. If they are different, remove the drive gear off both and install your '55 drive gear on the '57 tail-shaft. Now install the '55 rear extension housing on the '57 transmission. Once the trans has been installed in the car, you can plug-in your original '55 speedo driven gear into the hole in the extension housing and it will work as originally.
If you don't do any of this, the speedo cable will rotate backwards (if it rotates at all) and it will not register on the speedometer.
BTW, the speedo drive gear is held on the shaft with a big "C"-ring, which you can re-use to re-install the drive gear on the shaft.

Re: speedo gear-in tranny
 

It is a 57 FOM as the ring gear is on the convertor and not flex-plate.

The car you are working on is a 57, correct?

Can you show a photo to see if it is the old style? Might as well upgrade while it is out.

Re: speedo gear-in tranny
 

Dave, I am sorry for the misinformation but both trannys are 57 FOM's. So it will be a straight swap over after thinking about it. Sorry it was a giant brain fart on my part.

Mike

Re: speedo gear-in tranny
 

K yes it is a 57, I will shoot a pick of the flex plate tomorrow.

Re: speedo gear-in tranny
 

Then why is Dave describing a '55?