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how far off is my speedo

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by JMZ, Jun 2, 2002.

  1. JMZ

    JMZ Registered Member

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    I have 3.73 gears and 35 tires. I don`t know what the factory tire size was far these blazers.
     
  2. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I can't tell you how far off your speedo is but I can tell you that it works on a percentage basis and will not be the same at different speeds. I had mine calibrated at a speedo shop for about $100.00. I went from 4.10's to 4.56's with 35's and mine was off by 6%
     
  3. JMZ

    JMZ Registered Member

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    Yeah , I know its a %, I figured if it came with 31`s and I put 35`s It is 13% off, but I don`t know what they came stock with.
     
  4. rampage

    rampage 1/2 ton status

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    The easiest way is to have a friend drive in front of you on the highway. When they hit 60 have them tap the brakes and hold the speed steady. Check what your speedo reads and then you'll know the percentage you're off by. I just did this and found that mine is off by 30% /forums/images/icons/shocked.gif
     
  5. swede413

    swede413 Registered Member

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    head to wallmart and get a G.P.S. set it on your dash and drive around. it takes all the guess work out. If by the end of the day you dont think its the coolest gismo youve got in your truck you can allways take it back and get your money back.
     
  6. chulisohombre

    chulisohombre 1/2 ton status

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    mines got 3.73 and 35's.i got pulled over going 60 in a 50 so i guess that mines off by 10 miles an hour since i was going 50.yours is probably pretty close
     
  7. Steve_Chin

    Steve_Chin 1/2 ton status

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    Don't work off of the speedo. Set the speedometer gears up to make the odometer read correctly by driving a known length of road and checking against the odometer. Alter the gears in the transfer case accordingly (or get a ratio adapter to over/under-frive the speedo cable the right amount). Then calibrate the speedometer itself correctly (there should be an adjustment for the tensioning spring in the speedometer head).
     
  8. JMZ

    JMZ Registered Member

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    Thanks for all the replies.
     
  9. Michael

    Michael 1/2 ton status

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    Steve, if the same gear drives the odometer and the speedo, if one is calibrated wouldn't the other be also. I never heard of doing one then the other....or an adjustment on the "spring". /forums/images/icons/confused.gif
     
  10. Steve_Chin

    Steve_Chin 1/2 ton status

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    In theory, if you get the odometer calibrated, the speedo should be close. The speedos are never perfectly on, though. I calibrated the speedo in my Trans Am to be dead-nuts on at 120. It reads a little high under 70.
     
  11. Triaged

    Triaged 1/2 ton status

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    So...where did you do this calibrating at 120 /forums/images/icons/shocked.gif ?
     
  12. Steve_Chin

    Steve_Chin 1/2 ton status

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    On a dyno.
     

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