Spedometer Gear Question

Engine, Transmission & Driveline

  1. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    The drive gear is part of the output shaft, so it's not that.

    More likely, the speedo is just off. Check the odometer as was mentioned by comparing it to mile markers (do an average of at least five, better at ten or more).

    If the odometer is right, then the speedometer head is off.
     
  2. griswold00

    griswold00 New Member

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    How do we fix the spedometer head then?
     
  3. MrMoparCHP

    MrMoparCHP Senior Member

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    There are gauge repair shops out there,
    North Hollywood Speedometer
    Redline Gauges
    Are just a couple.


    Alan
     
  4. 3175375

    3175375 Senior Member FCBO Gold Member

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    They have been around a long time.
     
  5. RagTop66

    RagTop66 Member

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    Definitely put a pan under there, I thought I’d only lose a few drops and it was actually more like two quarts! Made a huge mess!

    If you figure out your rear gear ratio and need a different speedometer gear (which is what I’m suspecting) they sell them on Amazon for about $30 shipped. Much better than the crazy prices you’ll see on eBay. I pm’d you my cell, feel free to text or call me.
    Mike
     
  6. RagTop66

    RagTop66 Member

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    I’d be surprised if your tire size and/or rear end ratio aren’t throwing you off.

    Another possibility is the speedometer is gummed you with 54yr old grease. I’ve taken mine apart and re-lubed it and that helped fix mine.
     
  7. live4theking

    live4theking Old Man with a Hat

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    My speedometer is off because of the size tires I've got. They are about 3/4" shorter than they are supposed to be. To do 70mph the speedo had to indicate slightly more than 80mph. What car are you working on? What size tires did it have from the factory? What size tires are on our now? I plan to buy taller tires to fix my problem in the future.
     
  8. rapidtrans

    rapidtrans Senior Member FCBO Gold Member

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  9. griswold00

    griswold00 New Member

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    235/70/15's are on the back. I assume it is the default rear ratio for the Polara but that is a wild card with a used vehicle.
     
  10. RagTop66

    RagTop66 Member

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    Without knowing the rear ratio, you’re really guessing at which speedometer gear you need. Jack up your car and spin the back wheel by hand. Count the number of revolutions the drive shaft makes. The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio. For better accuracy, turn the wheel 20 times and divide by 2 for your rear axle ratio.
     
    Last edited: Aug 26, 2020
  11. RagTop66

    RagTop66 Member

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    I’m guessing you’re going to find 2.76 gears in your differential. Which with your current tires will call for a 26 tooth gear. I may actually have one, but I’ll have to check. I just swapped my old one out for a 32 tooth to match my tire/differential combo. My speedo is now spot on, where in the past it was off about 10 mph at 45mph.
     
  12. marko

    marko Well-Known Member FCBO Gold Member

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    did your car have 14" wheels when new ?
     
  13. 66 Monaco 500 365

    66 Monaco 500 365 New Member

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    Since the speeds are the same percent difference, just take the current tooth number and reduce it by 3/4
    If it has a 30 tooth in it, 30 *.75 it would be 22.5 call it 22
    My $.02
     
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  14. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    Those tires are about 2.5-3" taller than the 14" tires that were original, that and some old grease in there and you found your error.
     
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  15. RagTop66

    RagTop66 Member

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    Bottom line is figure out the rear ratio and use the chart. Do start guessing with $30-$50 speedo gears. Do it once.
     
  16. griswold00

    griswold00 New Member

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    Ok time for an update! :)

    I have the speedo out of the car and plan on taking it apart and lubing it up tomorrow. Personally the 27 tooth gear that is in the car currently is pretty close to the presumed correct 26 tooth gear. So I'm going to assume it's a speedo error and not a gearing issue.

    As it stands right now, the car is in a somewhat precarious position of being up on front jackstands on a slanted drive way. Yes the rear wheels are chocked :) As such jacking up the rear right now is not an option, so I won't be checking the rear gear until early next week. However we also won't be driving the car until early next week either, so it's not like we'll be able to test any hypothesis until the middle of next week either way :)

    I plan on determining the rear gear just so we know it going forward. I'll let everyone know how the speedo breakdown and lubing goes. I plan on doing that sometime tomorrow depending on how busy my work schedule is :)

    I appreciate the help from everyone, and all the feedback, I really don't know much about this stuff and appreciate all the input.
     
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  17. RagTop66

    RagTop66 Member

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    There’s a video in the link below that shows how to re lube the speedo. There’s a little brass plug that you can pop out and and remove a piece of felt to get lube to the inner parts of the gear.

    ‘66 Polara speedometer
     
  18. HWYCRZR

    HWYCRZR Senior Member FCBO Gold Member

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    If you are lucky the ratio tag is on the deferential hanging on one of the bolts.
     
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