Speedo reading wrong

Discussion in 'Got gears?' started by Dwayne B, Mar 16, 2018.

  1. Dwayne B

    Dwayne B Well-Known Member

    I went from A 2.56 to 3.42 ,my speedo is way off.Im thinking about using A speedometer gear reducer adapter,I need help finding the right ratio adapter .GM used these gear reducer adapters,on cars with 4.10 gears and so on.They made different ratios ,and I need help on which one I need.
     
  2. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

  3. 455monte

    455monte Well-Known Member

    I changed from a 2.93 to a 3.55 in my 67. And then swapped the th400 for a 200 4r.
    My speedo was reading faster than before.
    I downloaded an app that reads gps and gives exact mph.
    So i found out at 50mph on the app my car was reading 61 mph.
    Thats a hair more than 20 pct off.
    I dug through dads stash and found an adapter box that was rated at .800x ratio.
    After installing that my speedo is within 1 mph of correct and close enough for me
     
  4. BrianTrick

    BrianTrick Brian Trick

    Any time that you you increase your rear gear ratio,numerically,your speedo will read faster than it should. Just change the speedo gear in the transmission .
     
  5. 455monte

    455monte Well-Known Member

    I agree with brian.
    My only issue is i dont have any spare gears for the 200 4r!
    I used what i had laying around so i did not have to wait a few days for a correct one to come in the mail
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    You may not need an adapter. You may be able to just change the speedometer DRIVEN gear. To figure out what you need, you need 3 figures. One, you need the tire height (figured from rear tire size),two, the gear ratio (3.42), and third, the number of teeth on the DRIVE gear. The DRIVE gear is on the transmission output shaft. You can see it when you pull the DRIVEN gear sleeve on the transmission. It is a helical gear, like a distributor or camshaft drive gear. Pull the sleeve out. Then rotate the driveshaft (rear of car in air, trans in neutral). Count the teeth on the DRIVE gear. You may need another sleeve as well as there are two different sleeves, one for DRIVEN gears with 35-39 teeth and another for 40-45 teeth.

    Then put your numbers into this calculator,

    http://www.tciauto.com/tc/speedometer-gear-calculator
     
    Dwayne B and rkammer like this.
  7. Dwayne B

    Dwayne B Well-Known Member

    OK thanks guys
    I was just thinking about A gear reducer box because I'm going to put A 4 speed manual transmission in it in A few years.
     
  8. bigce1972

    bigce1972 Well-Known Member

    This is what I did. Speedo works fine!
     
  9. schlepcar

    schlepcar Gold Level Contributor

    Assuming it was accurate before and you are using the same tires you should be close enough with a 1 to 1.33 ratio. I am trying to remember how those gear transducers were marked. You basically need a 1/3 overdrive so the cable turns faster than it used to.
     
  10. Mark Demko

    Mark Demko Well-Known Member

    I have 3.42 gears and I had to change the drive AND driven gears in my T-350 trans, its now spot on, its read 10-11 mph faster than actual for 20 plus years, I got tired of "ok, 80 mph is really 70 mph"
    I used GPS after the gear change to check it and its on the money:D
    I posted about it awhile back, I forget what gears I used, but check the link Larry posted.
    This is where I got my gears fromhttp://www.transmissioncenter.net/speedometer_calibration_______va.htm
     
    Last edited: Mar 17, 2018
    300sbb_overkill likes this.
  11. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

    One of the characteristics I like about the 200-4R as opposed to all the others is that you can change speedometer the drive gear simply by dropping the pan and pulling the governor.
     
  12. john.schaefer77

    john.schaefer77 Well-Known Member

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