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    Cold Tire Pressure Temperature

    The manual says the "cold" tire pressure should be 39 psi for the 19 inch rear tires. What is the temperature of "cold"?

    Re: Cold Tire Pressure Temperature

    According to the owner's manual, the "cold temperature" is 68 degrees Fahrenheit (20 degrees Celsius).


    Jim

    Re: Cold Tire Pressure Temperature

    Thank you Jim. I should have read the manual more carefully.

    Re: Cold Tire Pressure Temperature

    Quote:
    Jim48 said:
    According to the owner's manual, the "cold temperature" is 68 degrees Fahrenheit (20 degrees Celsius).


    Jim



    Or for those of us without pyrometers--I check and adjust tire pressures after the car has sat overnight.

    Re: Cold Tire Pressure Temperature

    The rule of thumb is for every 10* Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

    Do the math from 68*, you'll be on target!


    Re: Cold Tire Pressure Temperature

    I do not think that there is a fixed cold temp. If you use as cold temp 20C/68F, that means that your tires will be underinflated on colder days and overinflated on warmer days.

    I do think that "cold" pressure means pressure at rest after sitting overnight in the shade. This implies that as the ambient temp changes one has to add air in the Winter and remove air in the Summer.

    Re: Cold Tire Pressure Temperature

    Quote:
    ADias said:
    I do not think that there is a fixed cold temp. If you use as cold temp 20C/68F, that means that your tires will be underinflated on colder days and overinflated on warmer days.
    I do think that "cold" pressure means pressure at rest after sitting overnight in the shade. This implies that as the ambient temp changes one has to add air in the Winter and remove air in the Summer.



    From the Owners Manual:

    - on Page3 of this Owner's Manual
    - and in the left door aperture.
    These values are for cold tires (68*F (20 *C)).

    Re: Cold Tire Pressure Temperature

    Quote:
    ADias said:
    I do not think that there is a fixed cold temp. If you use as cold temp 20C/68F, that means that your tires will be underinflated on colder days and overinflated on warmer days.

    I do think that "cold" pressure means pressure at rest after sitting overnight in the shade. This implies that as the ambient temp changes one has to add air in the Winter and remove air in the Summer.


    ADias, your 2nd statement answers your 1st statement. As the tires underinflated on colder days (1st), you need to add air in the winter (2nd).

    Re: Cold Tire Pressure Temperature

    I measure mine every time before I go out for a ride and adjust the pressure accordingly. However, that plays havoc with the resulting measures given out by the tire pressure monitoring system, if you have one on your car.

    It's too bad Porsche doesn't make up a printed table with the required pressure at different temperatures, for when the tires have achieved a temperature equal to that of the surrounding air. That should not be too difficult, as I think the formula is (P1 times V1)/T1 = (P2 times V2)/T2, where P1 = desired pressure, V1 = actual internal volume of the tire, and T1 = temperature of the air in the tire. That could be the standard level recommended by Porsche at the stated temperature of 68 degrees Fahrenheit. Porsche can calculate that base number, (P1 times V1)/T1. Unless the tire volume changes significantly (less likely for 18 and 19 inch tires), that volume could be the same for V2. Then with the actual air temperature, you can easily solve for the pressure level (P2) consistent with the recommended pressure at 68 degrees Fahrenheit (or 20 Celsius).

    Of course, they might have to recalculate the table for each wheel style, if the internal volume varies when a tire is mounted. It might also be likely that different manufacturers' tires hold different volumes of air, given a specific wheel. Jim

     
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