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Tire pressure question

Discussion in 'Gen 2 Prius Main Forum' started by jkash, Aug 2, 2004.

  1. jkash

    jkash Member

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    I have read all the posts about tire pressure. It is emphasized that the tire pressure should be checked when the tire is cold. Does this mean cold tire pressure in the morning when it is coldest or cold tire pressure in the afternoon when it is the hottest? I put air in my tires this morning. It was 64 degrees. I put in 40/38. When I checked them this afternoon, the tires read 44/42. They were in the shade, but I can't say if they had been all day.

    Any thoughts?

    Jeff
     
    #1 jkash, Aug 2, 2004
  2. randalla

    randalla Member

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    I believe that cold tire pressure/temperature means that the car should not have been driven for several hours giving the tires a chance to cool down to the ambient outdoor temperature. Ideally, your car should be in a shady location out of the sun so that the heating effect of the sun on your tires doesn't skew your pressure readings.
     
    #2 randalla, Aug 2, 2004
  3. efusco

    efusco Moderator Emeritus
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    Yea,
    "cold" refers to the not friction heated state. Ideally you want to check the tire pressure at the maximum daily high temperture, but with "cold" tires. This is esp. important if you're running at the maximum (i.e. 44/42). It's also why I choose to run at 42/40 so that there is some margin for error as I don't check every time the thermometer swings a little.
     
    #3 efusco, Aug 2, 2004
  4. jkash

    jkash Member

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    Given what you have said, I really shouldn't run at 42/40. The difference in the morning and after temps can be anywhere from 30 to 40 degrees. At 40 psi this morning, the tires were 44 psi this afternoon. 40/38 is the safest for me considering it is cooler today. We have seen temps in the 100's this summer and today is only in the mid-80's. Might even be wise to set them at 38/36 until the weather cools down.

    What do you think?

    Jeff
     
    #4 jkash, Aug 2, 2004
  5. randalla

    randalla Member

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    I'm running 40/38 in my tires. I'm not ridiculously fanatical about checking the tire pressure but I will do it about once a month while my car is parked in the garage after sitting for at least 3 hours.
     
    #5 randalla, Aug 2, 2004
  6. efusco

    efusco Moderator Emeritus
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    Yea, go with the lower numbers if your temp swings are that great....Jerry P can tell you exactly how much your pressure 'should' change for every degree of temp. change, but I've forgotten the conversion....was it ~1bl/10degrees F?
     
    #6 efusco, Aug 2, 2004
  7. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    ok cold temperatures should be after the car has sat awhile.

    running temperatures of tires can be very warm (35+ºC) but the STP (standard temperature and Presure) for gas measurement is 4ºC or about 40ºF. this is obviously colder than most mornings at this time of year nearly anywhere in the US.

    Now whether they use this measurement for the tire presure measurement i dont know. but either way i dont see why it wouldnt work. the STP calculation is made using absolute temperatures. so the air pressure between say 20º C and 40ºC and an air pressure of 38 PSI on the low end would be 41 PSI. that would assume a linear relationship increase and a difference in temperature where 20ºC would be 293ºK and 40º would be 313ºK making a difference of 7.3%

    ohhh kay... hope i didnt lose you... im not sure where i am right now to be honest with ya.
     
    #7 DaveinOlyWA, Aug 2, 2004
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