1. #1
    Jeepsrcool's Avatar
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    Yes, Another Speedo Question

    Ladies and Gents, I am having an issue with my speedo. 2006 TJ X model with 3:07 gears and running 33s at the moment. Before I would be about 6=10 slower than the speedo indicated and I changed out the factory speedo gear to what all the charts said to go to which was a 26 tooth. Now, I am 10-15 miles slower than the speedo reads. What gives? Any ideas folks?

    Thank you in advance and Happy New Year everyone.

  2. #2
    JPNinPA's Avatar
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    Yes, Another Speedo Question

    Are you certain you have 3.07 gears? If you have an automatic trans the stock gearing was 3.73 that would need 31 teeth. Another thing is to be sure you clock the gear properly when replacing. There is a plate on the case noting where to clock the speedo to to ensure the proper engagement.


    Regards,
    JPNinPA

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    Last edited by JPNinPA; 01-02-2018 at 11:41 PM.
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    Re: Yes, Another Speedo Question

    You could verify your rear gearing by doing the following:

    Tools needed:
    A flat surface, Masking tape , 4 feet of string , Chalk to mark the tire.
    Steps:
    Park on a flat surface.
    1.) Tape one end of the string to the bottom (6 o’clock) of the rear drive shaft.
    2.) Mark the rear tire where it touches the ground.
    3.) Now move your rig forward so that the tire make one full rotation
    4.) Count the number of revolutions the string made around the driveshaft. Be exact ti the 1/4 revolution.
    5.) Calculate the ratio by converting the fraction of the revolutions to decimal. Your ratio is x.xx:1
    Example:
    If the string is wrapped around the drive shaft 3 ½ times than you have the 3.50 to 1 gear ratio, which came stock behind a 302 V8.
    If you got the string wrapped around 4 ¼ times than it will most likely be 4.11 to 1 gear ratio.
    If it is wrapped 4 ½ times than it will be 4.56 to 1.
    If it is wrapped 4 ¾ times than it will be 4.88 to 1.

    If you don’t have room to move the vehicle, here is another way to do it. Instead of pushing the vehicle, just jack up the rear tires on jack stands and rotate it one full turn with vehicle in neutral.

    To be more precise you could rotate the tire 3 times and divide by 3 or rotate so the driveshaft ends on a full rotation and divide by the number of full plus fractional rotations of the tire.


    Regards,
    JPNinPA

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  4. #4
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    Re: Yes, Another Speedo Question

    Or look at the numbers on the tag under one of the diff cover bolts. Or pull the diff cover and get the digits off the ring gear.

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    Re: Yes, Another Speedo Question

    Yeah guys I got all that stuff and how to do it (not my first Jeep, but first with this problem). What I suspect is that the gears may very well be 3:73 instead of 3:07 as mentioned by JPN. Once it warms up this weekend, I will check that aspect out and I'll be damned if I rely on VIN decoder sites again. Checked three sites and they all said the same thing so I figured...

    Thanks and I will let ya'll know the results.

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    Yes, Another Speedo Question

    What was the tooth count on the speedo you removed?

    The delta from stock may also confirm the issue. Regardless of tooth count going from 30 to 33 in tires puts you in traveling 6.5mph faster than the speedometer reading at a reading of 55mph.
    If you had 3.07 gears that should be spot on using a 26 tooth speedo.
    If you really have 3.73 gears( 34 tooth) and switched to 26 tooth speedo the delta becomes 16.9 mph faster actual speed than the speedometer reading at a reading of 55mph.


    JPNinPA
    Using Jeepz.com app
    Last edited by JPNinPA; 01-04-2018 at 01:32 AM.
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    Re: Yes, Another Speedo Question

    Interesting JPN. That is nearly exactly what the differences were before and after the gear change except the numbers given in my original post were stated at about 70 mph. Right now, according to the GPS app, the speedo reads just under 85 and GPS reads dead on at 70. I will pay closer attention going home today at 55 and see what it says. Regardless, the numbers point to 3.73 gears don't they?

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    Yes, Another Speedo Question

    It's just a math ratio problem. So tire size alone before changing the speedo is 33/30=1.1. The 1.1*70=77mph. So when the speedo reads 70 you are a actually moving 77mph.

    Now with the speedo change you should need 31tooth speedo do 31/26 teeth used = 1.192 ratio. The 1.192*70 should give app83.5mph. Again when the speedo reads 70 you will actually be going 83.5 mph

    So short answer yes it points to 3.73 gears and it looks like you need a 31 tooth speedo. Just remember to clock it properly.


    Oh ya if you really did have 3.07 gears and put 33's on you would first be complaining about loss of power and not having enough to get you up to 70 in a reasonable time or without a downhill boost.

    Cheers!
    JPNinPA

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    Last edited by JPNinPA; 01-06-2018 at 12:00 AM.
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    Re: Yes, Another Speedo Question

    You nailed it JPN. 3.73s it is.

    I jacked up the rear, in N, and spun one wheel 10 times. That equalled approx 18.5 drive shaft rotations. 18.5 times 2 and divided by 10 gave 3.70. Close enough to be 3.73.

    Now it is time to order another speedo gear and NOT take the word of VIN decoder sites again without checking first.

    Thank you and everyone else that replied for your time and information.

    Andy

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