Changed Driven Gear in Transmission

[ICON 1956]

I just don't understand after I replaced the speedo cable with another OEM FO MO CO cable and a new driven gear (white 18 tooth)why the Speedo worked for a few months then quit, That being said I change pulled the speedo cable out to find that the wire inside the cable broke, ok so I replaced the entire cable I even attached to the drill to see if the digits would move yes they worked and I even replaced the white driven gear with a brand new one. I hooked up and now the dam thing still doesn't work...... Could anyone help with this ! Thanks. Going crazy.

[Karl]

G'day,
I am not to sure but I heard somewhere that there is a large nylon gear that drives the driven gear, if this is so maybe that has failed.
Maybe somebody else can chime in here and confirm this. I tried to find the gear in the M.P.C. but I couldn't find it anywhere.
Karl
P.S. I just found the part (I think)
Part number B7A-17285-B "Gear Speedometer Driving" it is a steel 8 tooth R/H helix gear 1.9" in Dia. I can't see how this would fail being steel that is if the one in you transmission is steel.
Karl

[ICON 1956]

Thank you for chiming in Karl, I will look into that steel gear.....Cheers!

[GeoffInCarlsbad]

I am having similar issues with my speedometer. I am planning to work on it this weekend.

Once I remove the speedometer instrument and detach the cable from the instrument, do I simply pull the interior cable out of the sleeve/housing? Or do I need to detach the bolt and clip at the transmission where the drive gear is attached?

[Karl]

Yes you can just pull the inner cable out of the outer without undoing anything from the transmission.
When installing it back into the outer cable just give the inner a twist as you insert it so that it goes into the transmission end properly. If it doesn't and sticks out of the outer to far then when you screw the speedo back onto the cable it will have pressure on the cable and speedo.
Karl.

[GeoffInCarlsbad]

Perfect. Thanks Karl!

[ICON 1956]

Yes I have pulled the inner speedo cable out and replaced it with another original one. But for some reason it dam speedometer still doesn't work

[Karl]

You could take the car for a drive with the speedometer out of the car and see if the inner cable is turning. you could apply a small amount of pressure to the cable with your fingers to replicate the small load the the speedo might have however I wouldn't apply to much pressure.
If the cable turns then the most likely problem is the speedometer. If the cable doesn't turn then there is a problem in the transmission.
Karl.

[GeoffInCarlsbad]

Yes, my cable does not turn. So, the plan is to replace the cable. It may be broken. Next will be the drive gear.

[GeoffInCarlsbad]

I pulled the Speedo cable, and it looks fine. Can use some new speedo cable grease, and I will do that. I then pulled the driven gear, and post pictures below to show the wear and tear over 54 years of driving.





I found the mounting bolt to be a 7/16", and it came our rather easily. Sorry the lighting is not the greatest, but those teeth are pretty worn, and just were not catching.

New 18-tooth driven gear on the way.

[ICON 1956]

My old one looks exactly like that ! , I bought a new one and installed in May of this year.. worked till early Aug . I pulled the inner cable to find that the inner cable was broken, I bought a new OEM in the FO MO CO box on E Bay only to have it replaced by the original one that I took out cause when I bought the car the speedo wasn't working. IFY before you go and spend $30.00 on a Driven gear from MAC'S I found the same 18tooth gear from Mustangs Unlimited for 12.95 free shipping. item # is F 1699R 02. You might want to check it out . Steve

[GeoffInCarlsbad]

My old one looks exactly like that ! , I bought a new one and installed in May of this year.. worked till early Aug . I pulled the inner cable to find that the inner cable was broken, I bought a new OEM in the FO MO CO box on E Bay only to have it replaced by the original one that I took out cause when I bought the car the speedo wasn't working. IFY before you go and spend $30.00 on a Driven gear from MAC'S I found the same 18tooth gear from Mustangs Unlimited for 12.95 free shipping. item # is F 1699R 02. You might want to check it out . Steve

That is awesome Steve! I will check that out. Thanks! I am hoping that my cable will hold up. It seems pretty strong right now. I will check out Mustangs Unlimited.

[GeoffInCarlsbad]

You can really see the difference between the new teeth and the wear on the old.

[GeoffInCarlsbad]

I actually just found this post:

As far as speedometer accuracy is concerned, it doesn't matter what the wheel diameter is, as long as the tire diameter remains the same. For instance, if you have 215/75 -14 tires, and you switch to 205/75 -14 using the same wheels, the speedometer wil no longer be accurate. If you get 15" wheels, 215/75 -15 tires will work.
As far as the width, it depends on the backspace, so "not messing with different offsets blah blah" might be disappointing. Some wheels designed for FWD cars have backspacing which probably won't work at all without wheel spacers, no matter how wide or narrow they are. I'm not 100% positive, but I believe 7" wheels with 4" - 4 1/2" backspacing is what you need. You also have to be careful about the size of the center hole, unless the wheels are "lug centric". If the center hole is too large, and the wheel uses cone type lug nuts, you'll need hubcentric spacers to prevent the lug holes from deforming.
If you search these forums, you'll find a lot of valuable and accurate info about wheels and tires.

So total diameter of the tire drives the accuracy of the speedometer. That makes sense. Is there somewhere that describes what was the original tire size of the 1961 T-Bird?

[tbird]

V.T.C.I OFS I.D"s. tire availability from the factory for each MY