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Speedo

Discussion in 'Differential & Driveline' started by Rob Goblin, May 20, 2020.

  1. Rob Goblin

    Rob Goblin Full Access Member

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    Stupid question, but I know nothing of transmissions or rear ends. Why is my speedometer off by 20+? It may actually be more. When I drive on the freeway, it's pegged out. In fact, it's pegged out driving on the country highways some times. Is it a gear ratio issue?
     
  2. Snoots

    Snoots Supporting Member Supporting Member

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    Cable driven or electric pulse? Not likely a gear ratio issue.
     
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  3. 82sbshortbed

    82sbshortbed Get outta there! Supporting Member

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    If it's cable, can't you buy different little gear things that go in the trans?

    I thought I remember you being able to swap them out when you put on different size tires. I don't know tho. Maybe wrong about that.
     
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  4. Rob Goblin

    Rob Goblin Full Access Member

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    I’m not sure. It’s a 1984, so I would imagine it’s cable, but I don’t know for sure. I have a parts truck, so if it’s the cable, I should just be able to swap those out correct?
     
  5. Snoots

    Snoots Supporting Member Supporting Member

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    Yes.
     
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  6. 78C10BigTen

    78C10BigTen Full Access Member

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    Yes, idr what theyre called. I have one hanging around
     
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  7. 82sbshortbed

    82sbshortbed Get outta there! Supporting Member

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    I thought so. Ha, I'm not crazy. Lol
     
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  8. 82sbshortbed

    82sbshortbed Get outta there! Supporting Member

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  9. Bextreme04

    Bextreme04 Full Access Member

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    Its different between a 2wd and 4wd. If you have a 2wd it will be in the trans and will be a colored gear that can be swapped out. That's what the amazon link above is for. If it is a 4wd, it is a bit harder. Generally you would have an adapter gear box with the ratio needed to make it correct screwed onto the output on the T-Case, with the speedometer cable screwing into that adapter. You can change the ratio inside the T-case, but it requires matching gears being changed and it's a pain. The amount of difference you are talking about would only happen if someone had it set up for a huge tire or low rear end gear, and then swapped in small tires or a really tall rear end gear. Like dead on with 35's and then going down to 28's or going from a 3.08 to a 4.56.
     
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  10. Rob Goblin

    Rob Goblin Full Access Member

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    It's a 2wd. I'm not sure what was done to it before I got it, but I'm pretty sure it may have been Frankensteined (if it was before, it definitely is now). It had aftermarket 15s with low profile tires when I got it as you can see here. I've since replaced them with the stock rallys and the stock tire size, but I really can't tell any difference in the speedo. Someone had mentioned to me that it could be the gear setup; that was why I asked here. As I stated in my original post, I know very little about transmissions or rear ends. I know my grandpa was always swapping gears in his farm truck depending on what he needed it for. If he was going to be using it as a daily driver, he would run particular gears, but if it was going to be used to haul cattle and farm equipment, he'd run different gears. He was a mechanic until he started farming. Unfortunately, he did not pass along his knowledge of vehicles to me.
     
  11. legopnuematic

    legopnuematic Supporting Member Supporting Member

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    sounds like either the cable or the speedo, it could be the spring that pulls the needle back has called it quits and is not holding the needle back, for it to be a gear issue it would have to be a huge change in gears to make it peg out like you describe
     
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  12. Bextreme04

    Bextreme04 Full Access Member

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    Agreed. I would lean towards speedo. There is nothing that could be wrong with the cable that would make it spin 50% faster than it should be.
     

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