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Thread: My speedometer is wrong, how can i get it fixed?

  1. #1

    My speedometer is wrong, how can i get it fixed?

    The owner of the car before me put larger tires on there. stock are 275's i think, and now i have 31's on there (not positive on the stock numbers, could be 295's, but i know they're much smaller). now my gear ratios and speedometer are off.

    i dont totally understand the gear ratio being off thing. so if somebody wants to explain that go ahead...

    but i know my speedometer is off, and i know why. anyway, the police put this thing that tells you how fast you're going in my neighborhood a couple weeks ago, and when my speedometer said i was going 18, the police thingy said i was going 21. thats a 6/7 ratio. so tonight my speedometer said i was going 90, so that means i was going 105

    thats a big difference... how can i get that fixed?

    also.. can somebody explain the gear ratio thing and how it relates to my tires?
    my car
    Originally posted by UnitedWeStand Don't go walking up to her and say "I just had a vasectomy wanna celebrate?
    Originally posted by EvilAngel Actually Kitten just got the sonagram done and the doc thought it was twins at first, but soon realized that it was the boys pen1s laying next to him....lol
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  2. #2
    SCSI Dude Faust's Avatar
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    gear ratio change from larger tires? that's a new one. not sure what that's all about. the gear ratios within the tranny as well as between between the tranny and motor as well as tranny and differential will be the same. no matter how big the tire size difference is. that the only drivetrain mod you have?

    with a larger wheel/tire diameter, there is more circumferance per revolution, thus the higher speed per motor RPM.

    how to get it fixed? i have no idea. try calling a Nissan dealer/repair shop.

  3. #3
    its a toyota

    and i heard something about i need to change the gear ratios... not the gear ratios changed
    my car
    Originally posted by UnitedWeStand Don't go walking up to her and say "I just had a vasectomy wanna celebrate?
    Originally posted by EvilAngel Actually Kitten just got the sonagram done and the doc thought it was twins at first, but soon realized that it was the boys pen1s laying next to him....lol
    Said by XSeanX on AIM I wish girls were a lot easier
    Originally posted by Needlefreak May the fleas of a thousand taliban camels feast happily on your lower region..
    Originally posted by Jim Heart attack on a bun?

  4. #4
    BE HAPPY ! ! ! ! ! poptom's Avatar
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    After second thoughts, I deleted my post because I wasn't sure what to do with the answers to the questions I asked.

    Anyway, you can make a mental calibration. I did that for years with a Ford I had, just remembering that 60 was really 65.

    Let's wait and see what some of the big-tire folks have to say.

    If there is a gear ratio to change, it would only be the little drive gear on the lower end of the speedometer cable. If it has one.
    "Mr President, you have big balls" - Dominica prime minister Eugenia Charles to Ronald Reagan after the invasion of Grenada, 1983

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  5. #5
    Token Dial-up User De Plano's Avatar
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    There is more than one set of gears in a car. The gear rayios your referring to are inbetween (well it is in the tranny)the tranny and the speedo. I have never done it but it should be a fairly simple thing for a shop to fix. You can call the Toy dealer and someone there should be able to tell you the gear you need given the tranny #'s and the tire size. That is unnecessary in a way though because unless your doing it yourself it would be cvered by the shop, if they can't find out what size you need then don't have them work on your car.
    By the way if you are a member of AAA they have a free service that lets you know how far off your speedo is and they will have suggestions about getting it fixed. You may be able to get a friend to say he is thinking of buying the car and get them to check if you don't have a membership. That would be dishonest though, and I do not recommend that, of course.

  6. #6
    Originally posted by De Plano
    There is more than one set of gears in a car. The gear rayios your referring to are inbetween (well it is in the tranny)the tranny and the speedo. I have never done it but it should be a fairly simple thing for a shop to fix. You can call the Toy dealer and someone there should be able to tell you the gear you need given the tranny #'s and the tire size. That is unnecessary in a way though because unless your doing it yourself it would be cvered by the shop, if they can't find out what size you need then don't have them work on your car.
    By the way if you are a member of AAA they have a free service that lets you know how far off your speedo is and they will have suggestions about getting it fixed. You may be able to get a friend to say he is thinking of buying the car and get them to check if you don't have a membership. That would be dishonest though, and I do not recommend that, of course.
    my parents have AAA and the car is fully under their name

    I'll look into that, thanks
    my car
    Originally posted by UnitedWeStand Don't go walking up to her and say "I just had a vasectomy wanna celebrate?
    Originally posted by EvilAngel Actually Kitten just got the sonagram done and the doc thought it was twins at first, but soon realized that it was the boys pen1s laying next to him....lol
    Said by XSeanX on AIM I wish girls were a lot easier
    Originally posted by Needlefreak May the fleas of a thousand taliban camels feast happily on your lower region..
    Originally posted by Jim Heart attack on a bun?

  7. #7
    SG Enthusiast twwabw's Avatar
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    Easiest way is to use a ratio adapter. First, you need to calculate the offset EXACTLY... here's a quick simple tool:
    http://www.atrol.com/calc.htm

    Then, just order an adapter. It will go inline, on the outside of the transmission, between the ferrule and the cable.

    http://www.gaugeguys.com/repairratio.htm
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  8. #8
    Court Jester RoundEye's Avatar
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    If it's off by 5mph, it will be off that much all the way across the speedometer. It won't increase the difference as the speed goes up.

    There are many ratio's in your drive train, let's go with the rear end. If you have 4:11 gears in the rear end, that means that for every 4 turns of the rear tire your drive shaft turns 11 times. When you changed the tire height, it changed that ratio. Now the speedometer gear in the tranny is spinning at a different speed.

    When you put a taller tire, it has to turn less to go the same distance, so it turns the speedometer gear slower, which results in your speedometer reading slower.

    Just the opposite happens when you put on a shorter tire, it spins more to go the same distance as before, that make the speedometer read higher.

    Maybe that's why the lowriders are always going so slow, those short ass tires have them thinking they are going faster then what they really are.
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  9. #9
    SG Enthusiast twwabw's Avatar
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    Originally posted by RoundEye
    If it's off by 5mph, it will be off that much all the way across the speedometer. It won't increase the difference as the speed goes up.
    Actually- not true. The difference will be smaller at lower speeds; greater at higher speeds. While the ratio is not variable, the indication is as it now becomes a percentage of difference.

    ie: 18 indicated, 21 actual yields a 1.166667 ratio. That differential at slower speeds is therefore much less than at higher speeds.

    indicated actual difference
    100 116.67 16.67
    90 105.00 15.00
    80 93.33 13.33
    70 81.67 11.67
    60 70.00 10.00
    55 64.17 9.17
    40 46.67 6.67
    30 35.00 5.00
    25 29.17 4.17
    Last edited by twwabw; 10-28-02 at 08:18 AM.
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  10. #10
    Court Jester RoundEye's Avatar
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    oops!
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  11. #11
    BE HAPPY ! ! ! ! ! poptom's Avatar
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    Originally posted by twwabw
    http://www.gaugeguys.com/repairratio.htm
    Good one. That looks like the solution if you really want to get into it. Dealer parts depts probably only have gears to accommodate different axle ratios rather than tire sizes. The gear is driven by a worm on the transmission output shaft.

    We had a Chrysler once where the speedo gear provided three different ratios just by rotating its mount. Very clever, I thought.

    Newer cars are probably all electrical rather than mechanical, though.

    If you need to measure the rolling circumference of a tire, you measure the distance between the axle center and the ground times pi rather than wrapping a tape around a loose tire.
    "Mr President, you have big balls" - Dominica prime minister Eugenia Charles to Ronald Reagan after the invasion of Grenada, 1983

    "We win and they lose. What do you think of that?" - Ronald Reagan, 1977

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