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Speed-o-meter cali-bration

Discussion in 'Wheels / Suspension / Tires / Brakes' started by jamesA, Aug 17, 2009.

  1. jamesA

    jamesA Well known pissed off telephone guy VIP

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    So I drove by one of those signs with a radar in it doing 35, and it said I was doing 32... do 205/50/15s make it differ by that much on stock Si rims or could the radar just be miscalibrated?
     
  2. |Chaz|

    |Chaz| Well-Known Member VIP

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    I've noticed my speedometer is always different than what those signs say. I wonder just how accurate those signs are though...
     
  3. eg6sir

    eg6sir Supa Mod Moderator VIP

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    could be your tires.. isnt the stock size 195/50?
     
  4. jamesA

    jamesA Well known pissed off telephone guy VIP

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    Thats what I'm asking, could the 205/50s make that 3 mile difference?
     
  5. Battle Pope

    Battle Pope New Member

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    At that low of a speed I don't think that a fractional increase in rotational distance would change that much. You'd have to be running 17s to see an appreciable change at that speed. More than likely it's the machine, not your speedometer.
     
  6. DESPAIN

    DESPAIN yep....mmhhm.......yep...

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    i have 17's on my prelude and my speed'o hasnt changed very much at all
     
  7. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    there are a few factors here:
    those speed signs are almost never calibrated well, ive seen them read 5 MPH +/- my actual speed
    speedometers usually read faster than your actual speed (65 in my mazda is actually 62 on a GPS)
    your OEM size is 195/55/15, your 205/50/15s will give a reading that is 1.6% too fast (speedo will read 60 MPH at 59 MPH)

    all in all i wouldnt be too concerned
    if you are doing the limit on your speedometer then you are not likely to get ticketed
    if you want to know for sure, check your speed with a GPS
     
  8. Bobby17772

    Bobby17772 New Member

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    I've got 205-40z-17 and the sign said I was doing 43 when my speedometer said I was doing about 35. I checked the conversion charts and it said there shouldn't be that much of a difference
     
  9. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    thats just your own damn fault for running 17s on an EF :puke:
     
  10. Bobby17772

    Bobby17772 New Member

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    it was a good deal, thats the only reason I got them. there was a little curb damage on one rim and they had 4 brand new tires, I only paid $80 for them. so I don't really think I lost. Personally I only wanted 15 but they were there so I grabbed them
     
  11. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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  12. jamesA

    jamesA Well known pissed off telephone guy VIP

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    Not too worried about it, just wondered if a small difference in tire size would take it out of calibration at all.

    It was one of those signs that the cops put up at the beginning of school with a radar on it to let them know to slow the fuck down in a school zone. I highly doubt they're calibrated that often if at all.
     
  13. newb

    newb phresh VIP

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    I thought the 205/50 would read slower on the speedo, not faster. The 205/50 is a slightly taller tire, meaning the car would travel farther per revolution of the tire with the 205/50, than the 195/50.

    Either way the difference would be very minor, shouldnt cause you any problems.
     
  14. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    the OEM size is 195/55 not 195/50
     
  15. newb

    newb phresh VIP

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    Ahh, I thought he said he had 195/50's. I see now, thats not what he said lol.
     
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