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Thread: Someone please school me on lifting and speedometers.

  1. #1
    My toys haul themselves. Sandbagger's Avatar
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    Default Someone please school me on lifting and speedometers.

    Ok so I have an 87 YJ with a mechanical speedometer AND and automatic transmission. I've replaced the YJ speedo with one from an early CJ7.



    So my question is: what will I have to do when I go up wheel and tire sizes? I'm told that this will screw with my automatic tranny. Do I need to replace gears in the sending unit or something?
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    Land Squid Jeffb_79's Avatar
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    I'm not sure of yj's, but tj models other than the rubicon have a speedo gear that just needs to be replaced.
    That, or change out the gearing in your differentials.

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    Junior Member 913's Avatar
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    Most Jeeps from mid-70's through mid/late 90's have mechanical gears to adjust the speedo. Let me know the details on your combo and we should be able to find you the right one.
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    My toys haul themselves. Sandbagger's Avatar
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    OK! thanks! I'll either be getting 30 or 31" tires.
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    Senior Member HillBillE's Avatar
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    And the speedo gear is in the T-case, what's in the YJ? A 231? Should be able to find a gear for that.

    My buddy has the formula for finding the right tooth gear, since not all tires are 'true' in there diameter. The formula will find the best tooth speedo gear for the tires you have on the Jeep.

    So, you do this: (you need a GPS to do this, as well as having your speedo working)

    speedometer reading x current gear tooth count, devided by actual speed = what you need
    example, mine, 73mph x34tooth gear, devided by GPSed 65mph= 38.1, or 38 tooth


    I have mounts for my GPS and a 12V power cord in each of my Jeeps. My CJ is close on the speedo with the 4:10 gears and 33" tires, but my XJ is off like 15%.


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    transplanted texan d-rail's Avatar
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    i stole this chart from another forum. hope it helps..
    from ohio yj on quadratec jeep forum.
    When you change to larger or smaller tires or change the gear ratio in your axles your speedometer will no longer read correctly. To correct this you need to purchase a new speedometer gear. Replacing a speedometer gear doesn't take any special tools and can be done in a matter of minutes. Refer to this chart for selection of the correct gear:


    numbers on left are your gear ratio, and the numbers on top are tire size.

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    My toys haul themselves. Sandbagger's Avatar
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    so let me get this straight.

    1) I believe I have 27" tires (not even on the chart :P).

    2) the gear RATIO is determined by comparing the overall size of the tire to the size of the gear.

    3) So when I increase the size of my tires, I'll need to increase the gear size in order to maintain roughly the same ratio of gear to tire.

    4) the gear ratio should roughly... 4.5? Or do I take the size of the current gear & tire and try to get that ratio?
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    Senior Member HillBillE's Avatar
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    The gear ratio is the ring and pinion in the axle, the tire size doesn't figure into the ratio mathematiclly.

    When you go with bigger tires, it's best to go with a lower gear ratio (which is higher numerically)

    You can 33" tires on a 3:55 gears, but it would be better to regear to 4:10 or 4:56 (higher number=lower gear ratio)

    Until you know the gears you will have, and the tire size you're going to run, you can't figure out which speedo gear you need.

    The charts like d-rail posted will get you close. ie: If you have 4:46 gears in the axle, and run a 33" tire, you will want a 39 tooth speedo gear.

    But, since tire size varies by manufacturer, ie: some 33" tires measure 32", some measure 33.5", the chart will get you close.

    Using the formula I posted will get you right on, once you have the gears and tires you are going to run.


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    Senior Member SavageSun4x4's Avatar
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    As you can see its complex and while not too $$$ not so cheap either. Then there is the hassle of it all,

    GPS units are so cheap and give very accurate mph readings, don't care what size tire you run or axle ratios or anything else. They just chart your movement across the globe. Get a nice Garmin for $125 or so new at Costco/Sams or buy used off ebay.

    I got my Garmin 1490 T (I think that was the model) for $159.95 at Christmas at Costco. Works in Europe and can read out in Km if need be...
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  10. #10
    Senior Member HillBillE's Avatar
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    Quote Originally Posted by SavageSun4x4 View Post
    As you can see its complex and while not too $$$ not so cheap either. Then there is the hassle of it all,

    GPS units are so cheap and give very accurate mph readings, don't care what size tire you run or axle ratios or anything else. They just chart your movement across the globe. Get a nice Garmin for $125 or so new at Costco/Sams or buy used off ebay.

    I got my Garmin 1490 T (I think that was the model) for $159.95 at Christmas at Costco. Works in Europe and can read out in Km if need be...
    Garmin E-trex, $75:


    That's what I use in my XJ, I have a Garmin Rhino 120 in the CJ. Toss in another $15 for a cord to run from vehicle power, and your good!

    Even if you change gears, or change tire size, the GPS will be DEAD ON accurate.


    As long as the people remain armed,
    government knows that it cannot rule over the people by force.
    Those who stand in defiance of unconstitutional laws
    do so out of duty, honor, oath and love of country.


    "Certified Jeep Junky!"


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