Speedometer Gear gets mangled after a short drive

[NOT A T5]

Hi All, replaced my speedometer gear (16 tooth) for the third time in just one week! Gear gets kind of carved out on one side ever so slightly until it won't be able to grab anymore. Could the driven gear be causing this? 302 with a C4. Thanks for any help!

 

[NOT A T5]

Found a picture!

 

[Cleveland Crush]

Ensure your speedometer cable is greased and not kinked in route up to the instrument panel. It should turn freely by hand while rotating the speedometer cable gear. Looking at your gear photo it may be time for a new tranny drive gear also.

 

[NOT A T5]

Graphite lubricated cable isn't kinked, spins freely and threw some Type F on the speedo gear! Thanks Cleveland Crush...how much time and work is involved in replacing the drive gear? How many teeth are on the drive gear?
By the way the car's rear end is 2.79

 

[Cleveland Crush]

I'll let the auto trans members chime in on your C4 drive gear replacement difficulty, I only know how to drive and work on manual transmissions but FWIW I typically use a Pink 6 tooth gear with or Black 7 in my Top Loaders with the appropriate speedo gear to adjust for any particular rear end gear and tire combination.

 

[Hemikiller]

The drive gear on a C4 is machined into the output shaft. Hard to say why yours are getting chewed up. I would suggest buying a Type 3 with the extended pilot tip, which is what certain applications of the C4 calls for. That part # is D0AZ-17271-B. You can get them at your local Ford dealer for a few bucks. The 16 tooth version has extended teeth as well.

https://www.fordpartsgiant.com/parts/ford-gear-driven_doaz-17271-b.html

 

[basstrix]

I wonder if your drive gear is incompatible with the number of teeth on the driven gear and it's destroying itself?

This is Don C's spreadsheet. It may offer some info. At one time, I found a compatibility chart, but I wasn't able to locate it.

https://7173mustangs.com/threads/speedometer-gears.15595/

 

[NOT A T5]

@Hemikiller...thanks for the info but their gears start at 1980 but I think 1970-1980 C4's are all the same!?

 

[NOT A T5]

@basstrix...thanks! I was using an off white 16 tooth speedo gear because my speedometer is only 2-3 miles off with it and my 295/50's! I had an 18 tooth speedo gear in it but that one was off by 7-10 miles.

 

[Hemikiller]

@Hemikiller...thanks for the info but their gears start at 1980 but I think 1970-1980 C4's are all the same!?

You can just go to a Ford dealer and ask for the part #, no need to use an application. The link I provided is just an example for pricing as some places charge $10 or more for a speedo gear.

 

[NOT A T5]

Yeah, I just got back from the local Ford dealer and they go back to the early 90's. So I will use the link you provided and order 3 of them, just to have a few extra...their shipping is almost $10.
Thanks so much!

 

[Don C]

One other possibility is that your cable is slightly too long and when the housing nut is tightened up it is forcing the cable into a bind. It appears that something is causing a lot of stress on the teeth on the gear. The output shaft gear should not be getting chewed up by anything, and causing the problem, the only thing it rides against is the plastic driven gear. Just be careful about not getting any dirt into the transmission housing when replacing the driven gear.

 

[NOT A T5]

Thanks for your input Don C.