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348 speedo question

Discussion in '348/355' started by tcannon, Jul 23, 2009.

  1. tcannon

    tcannon Formula 3

    Feb 18, 2009
    1,740
    Norman, OK
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    Todd Cannon
    Hello all, how accurate are your speedos in your 348s? I am noticing that around 25 up to around 60 or so, my speedo is about 5 mph faster than it should be. They had one of those speed radar trailers set up in my neighborhood and I was doing some testing with it (hopefully I will not receive a load of tickets in the mail!). My Honda CRV was spot on but the 348 was hitting 35 when the speedo was reading 40,41 or so. I tested this up to around 60 before I started to worry about the tickets. :D What has been your experiences? Thanks
     
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  3. AceMaster

    AceMaster Three Time F1 World Champ

    Feb 6, 2009
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    Mike
    from what i understand, the speed radar traps are not 100% accurate, but with that said there is about a 5mph tolerence.
     
  4. tcannon

    tcannon Formula 3

    Feb 18, 2009
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    Todd Cannon
    I also went out and had my girlfriend follow me in her Honda. I got the same 5 mph difference between 20 and 60mph. Not a big deal to me. I take it as part of getting to know my car. I just wondered if this was a common thing. Thanks
     
  5. SonomaRik

    SonomaRik F1 Veteran
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    Dec 15, 2006
    6,715
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    5+ MPH is about average.....good news is: your car is going 5MPH while parked :D

    aren't the Ferraris awsome ;_)
     
  6. Saint Bastage

    Saint Bastage F1 Rookie

    Jun 1, 2007
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    Todd...Do you have stock tires? The speedo counts revolutions at the gearbox. Tires with a differant circumference will register differantly. Smaller circumference is slower than indicated, Larger = faster. You can notice an appreciable differance with worn tires Vs. new.
     
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  8. tcannon

    tcannon Formula 3

    Feb 18, 2009
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    I never thought about that. You are right. I should have checked that. I will check the tire size and see what is going on. That is why I was asking the question. I was trying to see if this is normal or should I be looking for a cause. Thanks! Todd
     
  9. Saint Bastage

    Saint Bastage F1 Rookie

    Jun 1, 2007
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    FYI....any reasonable GPS system can also be used to get a sense of Speedo Calibration. Wouldn't use it when High accuracy counts...but to get an Idea...Perfect.
     
  10. SonomaRik

    SonomaRik F1 Veteran
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    Dec 15, 2006
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    that's one big circumference....is it really that far off on most normal tires for his model?
     
  11. Saint Bastage

    Saint Bastage F1 Rookie

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    #9 Saint Bastage, Jul 24, 2009
    Last edited: Jul 24, 2009
    About a 2% differance for every 1/2" change from stock diameter. Assume fresh stock tire for speedo calibration datum. Fully worn tire vs. fresh = about 1/2" diameter differance. Not a big deal, but change to a differant tire size and you can easily see 5 or 6%. A primary reason its difficult to get tire shops to install differant tires on cars with ABS systems. It supposedly screws up that calibration also. (or so I've been told)

    25" diameter = 78.540" travel per rotation
    24.5" diameter = 76.969"
    24" diameter = 75.398"
    23.5" diameter = 73.827"

    Put aftermarket 20" stuff on Wifeys Yukon and used speedo 60MPH as Highway datum. Calibrates out according to the nav system. She hated them so we went back to stock. (uuugglllyyy)
     
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  13. tcannon

    tcannon Formula 3

    Feb 18, 2009
    1,740
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    Todd Cannon
    You were right Saint Bastage. Here are the sizes of my tires verses stock. This creates a 2.6% error in the speedo. I found a great website that calculates everything for you at http://www.miata.net/garage/tirecalc.html . Thanks again for the feedback. I just need to keep this in mind while cruising down the road. ;)

    1991 Ferrari 348 TS

    Stock tire size:

    Front: 215/50-17
    Rear: 255/45-17


    Actual tires on car:

    Front: 225/45-17
    Rear: 265/40-17

    :)
     
  14. Saint Bastage

    Saint Bastage F1 Rookie

    Jun 1, 2007
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    Cool calculator...thanks for that
     

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