Spedometer Gear Question

Engine, Transmission & Driveline

  1. griswold00

    griswold00 New Member

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    Hello All,

    We bought a 66 Polara 383 2Dr Hardtop for my son about a year ago. We bought it at an auction and it was stated it had a "New 727 Transmission". So take that for what it is worth.

    When we got the car, the speedometer was not operational. We assumed they never ran the cable after the tranny swap. However after buying another cable, we found the old cable there, just not plugged into the transmission. So we plugged it in and viola a moving spedometer.

    However it doesn't seem to be geared correctly, as it reads about 26 MPH when we are actually going 35. Likewise at 45 on the spedo we are actually going 57ish.

    Does anyone have any advice on how we can correct this? I know there are different number of teeth on spedometer gears, but I'm clueless on how to get to the gear and take it out to count.

    Thanks in advance for any advice.

    Scott
     
    Last edited: Aug 25, 2020
  2. jake

    jake Senior Member

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    Do you know if the rear gears where change out?
     
  3. griswold00

    griswold00 New Member

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    I don't know for sure, but it wasn't mentioned at all, so I'm going to guess no.
     
  4. rapidtrans

    rapidtrans Senior Member FCBO Gold Member

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  5. 3175375

    3175375 Senior Member FCBO Gold Member

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    It can likely be corrected by changing the speedometer gear that drives the cable.
    Pardon my ignorance in specifically how it’s mechanized, but there are several gears available to adapt for different rear axle ratios, etc.
     
  6. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    TCI has a nice speedo gear chart with tire diameters not the old size chart.
     
  7. 413

    413 Well-Known Member

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    Take yours out and see how many teeth are on it

    DC864600-BA37-4709-8CAE-3F0CE12CD3AC.jpeg
     
  8. griswold00

    griswold00 New Member

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    Do you have to drain all the transmission fluid in order to get the speedometer gear out? Sorry, stupid question from stupid person :)
     
  9. MEV

    MEV Active Member

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    no, you may get a few drops. I jack the rear of the car up on an angled drive way just to be sure.
     
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  10. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    In my experience, the chart gives you a place to start, but the best I've been able to do is see what speedo gear you have now first. Since you are about 35% off, you'll want to change the gear 35%. This allows for any inaccuracies in the speedometer head etc.

    Compare the numbers with this chart. Look at what you have, check the chart and then figure it out from there.

    chart.jpg
     
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  11. griswold00

    griswold00 New Member

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    Thanks for all your help everyone! Looks like I know what the boy and I are doing when he get's home from school! :)

    I plan on removing the gear we have, comparing it to the chart 413 uploaded to determine the number of teeth we have currently. Then I'll order a new gear that has about 35% more teeth than what I have currently (per Big_John's advice).

    Thanks again everyone!
     
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  12. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    I think (and someone please correct me if I'm wrong) that you want less teeth.
     
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  13. 3175375

    3175375 Senior Member FCBO Gold Member

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    When I dialed in the speedometer on Tigger, my 65 Mustang after changing the rear end, I did a mileage marker run, traveling at 60 mph. I then created a spreadsheet to determine the gear that I needed to get as close to true, but reading slower than truth.
    IIRC, I got within 2-3%.
     
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  14. 3175375

    3175375 Senior Member FCBO Gold Member

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    FYI, the gears for my Mustang were not that expensive, so I bought all of the tooth varieties offered.
     
  15. griswold00

    griswold00 New Member

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    My logic on more teeth is that if we assume the cable spins 1 time for each tooth, and we assume it currently has 25 teeth, that means the cable is spinning 25 times per 1 rotation of the gear. Now, at 25 miles per hour on the speedometer, let's just say the cable is spinning 25 times. Well since we are actually going 35ish when the speedometer says 25, we want the speedometer cable to spin more times every time the gear spins 1 time, thus we want more teeth.

    That being said, that's just my weird non-car guy, no-experience whatsoever thought pattern. I've been told many times before I'm thinking of something in a wrong way and I probably am now as well.

    :rofl:

    Scott
     
  16. MrMoparCHP

    MrMoparCHP Senior Member

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    This is what I was told as well, the best way to calibrate a speedometer is getting the odometer correct over a long distance (50-100 miles)

    Alan
     
  17. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    Easier to check MPH against a GPS these days.

    The thing is that the odometer can be dead nuts and the speedometer can be off. The odometer has a direct connection to the cable and the speedometer has a spinning magnet that pulls the needle up. There is an adjustment on the speedometer (never done one) that can compensate.
     
  18. 3175375

    3175375 Senior Member FCBO Gold Member

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    You are correct in understand how it works. There are 2 gears. One inside the transmission, which sometimes is referred to as the speedometer drive gear and one on the speedometer cable side referenced as the speedometer cable gear.
    I am ignorant on what these are called in MoPar-land (think sub-frame vs stub-frame), but the function is the same.
    In Fords, one could choose either gear, albeit the one inside the transmission requires disassembly.

    The speedometer cable only rotates enough to move the indicator inside the dash (which isn’t directly coupled to the cable - it’s magnetically coupled to a rotating assembly (which is directly tied to the cable) and the needle has a cup, called a speedcup that shrouds the spinning magnet, which is all buried in the speedometer ‘gauge’).

    I hope that this helps.
     
  19. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    You need less diameter to spin faster and raise the mph. That would be less teeth. In a Mopar you do not change the drive gear, it is machines into transmission shaft, you only change the driven gear. Less teeth turns faster, this spinning cable faster and higher indicated mph on guage.
     
  20. griswold00

    griswold00 New Member

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    Well we got the gear out, and it's not really a color, it's kind of a clear plastic. Counting the teeth yields 27. So if I were to go with less teeth by 35% would get me down to 21-22 teeth. I am having a hard time finding a gear smaller than 26 teeth online. All this makes me think the drive gear is messed up as 21-22 teeth for the driven gear seems really low.