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Speedo Gear Change

Discussion in 'TJ How-to Guides' started by Chris, Oct 31, 2015.

  1. DN EG 3

    DN EG 3 New Member

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    It's a 97 Sahara with an automatic transmission. I'm the third owner, according to the guy I bought it from. I believe it's been modified due to not getting the correct parts, to replace my rear drive shaft u-joints, when I told the parts guy that it is a 1997 Jeep Wrangler Sahara 6 cylinder.
     
  2. PStov98TJ

    PStov98TJ TJ Enthusiast

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    It's normal for it to be off by a greater amount at higher speeds, because the gears all work as a ratio, which means after gear change and/or tire change, our speedometers are "off" by a percentage. Thus, it's off by 10% due to a change in gears or tires by 10%, you'd be 1mph high/low at 10mph, but 7mph high/low at 70mph.
     
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  3. Chris

    Chris Administrator
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    Then yes, the 1997 automatic models had an option of 3.55 gears, so you hit the nail on the head.
     
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  4. DN EG 3

    DN EG 3 New Member

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    HOLY SPEEDOMETER GEAR!!!

    The local dealer said the part is no longer available, but it would have cost me $142 if they had it.

    I don't need to know how fast I'm going that badly!
     
  5. Chris

    Chris Administrator
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    Amazon has the one you need for $32 plus free shipping:
    https://www.amazon.com/dp/B00EDWL6AY/?tag=wranglerorg-20
     
  6. DN EG 3

    DN EG 3 New Member

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    $
    $34 at Morris 4x4. That's not too bad
     
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  7. Kyle_W

    Kyle_W Artist formerly known as "Nashorn101".
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    I've been curious about this as well - since tires aren't manufactured to PERFECT specs (33" tires being more like 32" tires, etc.) - how can you really know what size your tires are?
     
  8. Chris

    Chris Administrator
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    It should tell you exactly how big they are on the manufacturer website.
     
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  9. eddiefast

    eddiefast New Member

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    great info and shared expertise. But a question: My build sheet for 98 TJ SE says 4.11 gear ratio. I always see 4.10. Why? In any event I assume I just go with the 4.10 for this but still wonder about the ratio delta
     
  10. Chris

    Chris Administrator
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    It's the same. Some of us just call it 4.10 while others call it 4.11, but they are identical. Not sure why people started calling them by one name or another.
     
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  11. IamBR549

    IamBR549 TJ Enthusiast

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    Next project on list!


    Sent from my iPhone using Tapatalk
     
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  12. JamesAndTheSahara

    JamesAndTheSahara TJ Enthusiast

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    I just did mine! Beware, that stupid o ring from the dealer is $10 ea and is backordered currently in the SE. When I removed mine and my peers, both of our orings were in fantastic shape. Make sure whether you reuse or install a new oring that you apply a little bit of ATF+4 around it so it does not tear going in. In the event you go the Stu Olsen route and go to a hardware store to source an oring, you must use a hydraulic oring.

    Goodluck!!
     
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  13. BumbleJeep

    BumbleJeep New Member

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    OK... I'm really confused :) My speedo was reading high, so I bought a new speedo gear - my gear ratio is 3.07, I'm running on General Grabber AT2's in LT30x9.5, so I bought and put in a 28 tooth gear. Now my speedo is high by even more! I'm reading 8mph over actual mph...

    very confused...
     
  14. Chris

    Chris Administrator
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    You sure you have 3.07 gears?
     
  15. BumbleJeep

    BumbleJeep New Member

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    yeah, 3.07 tag, and revs match what a 3.07 would do at speed with tire size. very weird, like someone put a speedo healer in-line instead of a gear change?
     
  16. Chris

    Chris Administrator
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    Since the speedometer on those things is gear driven, I am fairly certain you couldn't use a speedo healer, since it's electronic.

    However, how much faster is it saying you're going at any given speed (how much is it off by)? What did you use to determine this?
     
  17. BumbleJeep

    BumbleJeep New Member

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    Thx Chris! I use a Garmin GPS to determine speed - right now 80 indicated on the speedo is 73mph actual. As you'd expect, in the 20's and 30's it's only a couple/few MPH off. The error grows as you go faster. The only thing I can think of is that someone put an electronic adjuster on it somewhere, I'll look more this weekend.
     
  18. JamesAndTheSahara

    JamesAndTheSahara TJ Enthusiast

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    To clarify, the only way an electronic healer could be used & work is if you have a tone ring. Since you physically installed a speedo gear its time to pull it back out & recount the teeth on it to make sure you are correct on the chart & sold what was advertised as a 30 tooth gear. I wouldn't trust revs or tag on terms of gear ratio unless you are the original owner. I would pull the cover & read the gear indicators to know 100%. Because its mechanically driven its hard to see why it would be 7mph off if the gear is correct for the tire size.
     
  19. Chris

    Chris Administrator
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    X2 to this. Pull the gear out and count the teeth. Also, there’s a very real possibility you don’t have 3.07 gears. Someone could have re-geared it.
     

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