# Tire pressure......cold vs warm tire?

robarSeptember 29, 2013

On my vehicle all I have to do is press a button on the dash and it shows my front and rear tire pressure. The truck requires a tire pressure of 35. I checked them when the tires were "cold" and the reading was 33......but after driving a while and the tires "warmed up"....the reading was 35. Does tire pressure read less when tires are cold? Is the pressure okay, since it came up to 35 after I drove for awhile? Any suggestions will be appreciated.

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jemdandy

Yes, tire pressure read less when cold. In the vicinity of this temperature and pressure, air closely follows perfect gas behavior. In other words, P1 x V1/T1 = P2 x V2/T2
Where P1, V1. and T1 are pressure, volume, and temperature at initial conditions and P2, V2, and T2 are pressure, vloume, and tmeoerature at some other condition. IMPORTANT: Temperatures must be in absolute degrees, ie, measured from absolute zero. Absolute zero for celsus is -273 C and for Farenheit, it is -460 F.

Over a working range of 30 to 50 psi, automotive tires do not change volume by a significent amount.

We have the following relationship:

P1 V1/T = P2 V2/T2 For our tire problem, the volumes effectively cancel leaving:

P2 / P1 = T2 / T1

Solving for temperature change given the pressure change:

Shuffling the preceding equation gives:

T2 = (P2 / P1) T1

For example, assuming that you tire was 60F (520 R) when cold, and given that the 2 pressures were 33 and 35 psi, we have

T2 = (35/33) 520 R = 1.0606 x 520 = 551.5 R or 91.5 F

[ add 460 to deg F to get absolute temperature.]
[ add 263 to Deg C to get absolute Temperature on the celsus scale.]

As you can see, pressure does vary with its temperature. In summer, I assume that the cold temperature is 60 to 70 F and inflate to the standard recommended pressure. When the tire is hot, the pressure will be higher.

Knowing how the pressure changes with temperature, we can expect the pressure to fall in cold weather. The pressure change from 60 F to 20 F is 0.923 P1. A tire inflated to 33 psi at 60 F would read 30.5 at 20 F

So, should you inflate to full presure in winter? Maybe. It all depends. If you expect the tire to go up to 100 F in operation. then inflate a bit on the low side. However, it has been my experience that properly inflated tires do not gain a lot of temperature in winter. At 30 F, I inflate my tires to near the recoomended pressure ( That's the pressure on the door sticker, not the max. pressure on the sidewall of the tire. The max. pressure on the sidewall of the tire is not its working pressure, but is the max pressure the tire should ever see. Some tire shops will take the tire up to its maximum pressure when mounting a tire and as soon as it has seated on the rim, the pressure should be reduced.

When the tire has been pressurized beyond the sidewall marking, it can be damaged and sometimes this damage is not apparent, but may show about 6000 to 8000 miles later. Broken or stretched belts may show as a worn out center portion of the tread. Often, the tire shows unbalance at highway speed and the tire shop will not be able to balance this out becasue the tire changes shape at speed. Its a dangerous tire and should be discarded.)

September 30, 2013 at 3:54AM
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klseiverd

In simple terms... almost everything expands when heated and contratcs when cooled. So cool/cold tires would show lower pressure than warm/hot ones.

October 11, 2013 at 2:26PM
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jemdandy

Uh-oh, there's a mistake in the above calculations. Absolute zero on the celsus scale is -273 C, not -263 C. Sorry about that.

October 11, 2013 at 4:27PM
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mxyplx

HooRay for Charles and Boyle.

Shuffling an equation? :-) Excellent! Excellent!

Been rearragning em since I was 13. From now on they will be shuffled.

October 14, 2013 at 10:48PM
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