The door sticker says 30 psi - is that at sea level or does elevation not matter? What about outside temperature- how does that effect tire pressure?

 

Boyles and Charles law tell us that temp and air pressure effect tire pressure. High up there is less pressure on the outside of the tire so there is more on the inside.
The higher the temp the faster the atoms/molecule move resulting in more pressure inside the tire.

 

When a tire gage reads the right pressure, stop. It's the difference between the inside and the outside.

 

For every 1,000 feet in elevation above sea level, atmospheric pressure decreases about a pound and a half. As a result, tire pressure goes up an equal amount.

 

BUT if the pressure gage is at the same atmospheric it should read correct.
mwagne16 is correct

 

HOLY **** I could have died When I drove up pikes peak 32# at sea level when I left +14000 ft 1.5*14=21 ending tire pressure 53 pounds Do I need new tires? The max pressure says 44#

 

If they did not explode at the top, I think you are alright.
After all, the guys at the tire place think putting 50 lbs. in your tires is normal!!

Also, as you went up, the temperature probably was dropping, so that would tend to make the pressure lower. Not much you can do about it. If you bleed pressure off at the top, when you get back down, the pressure is 11-12 lbs. I would rather have the pressure a little high, rather than the pressure being too low.

 

It's not linear though.

Sea level air pressure is about 15psi.... so if you filled tires to 30psi at sea level, and then launched your car into space (near vacuum), the max pressure they could read is 45psi, assuming the air temp inside the tire stays the same.

So you won't exceed the max pressure rating by driving to the top of any mountain if your pressure reading is 30psi at sea level.

 

Tires are a closed container, if you have 30 psi at sea level, you still have 30 PSI in the tire at 50,000 ft MSL. The total volumn of air within the tire has not changed.
The main thing that changes is the difference between the inside tire pressure and the outside atmospheric pressure.
Temperature changes affect tire pressures, not altitude.
The tire retains it's shape and size because the tire casing is a reinforced casing, it does not expand like a rubber balloon.

 

Maybe i should stop every 1000 feet and re adjust

 

Ehhhh.... no.

It's true that tires are a closed container, but a tire guage will measure the differential air pressure only.

Think about this: if you mount a brand new tire, but don't inflate it, what is the tire pressure? Technically it's about 15psi (atmospheric pressure). But the pressure inside and outside is the same.
When you use a tire guage, you are creating a pathway for the air inside the tire to flow outside. But when the pressure is equalized, there is NO net pressure against the spring in your tire guage. It will read zero. If the tire guage read anything other than zero (which requires a force against the spring in the tire guage), you have violated some laws of thermodynamics, and you'd be able to build a perpetual motion machine.

Now, inflate your tire to 30 psi at sea level. Put the tire inside a hyperbaric chamber (pressure chamber) and pressurize it to 30psi.
What does the tire guage measure? Again, when it presses open the valve stem of the tire, there is no airflow out of the tire, so the net pressure is zero.

Take the same tire and put it in outer space. The tire guage will read about 45 psi, because you no longer have atmospheric pressure pushing back against the other side of the little piston inside your tire guage.

 

I agree with you in that the normal tire guage measures the differential pressure. They are probably calibrated against 29.92 inches of mercury. Since that is the average air pressure at sea level, for a standard rate day, that calibration factor should work well enough in most places.
So, with that in mind, how do tire pressure monitoring devices work when they are mounted inside a tire. Well, they are calibrated against a standard pressure, (29.92 you think). OK now, hold that thought. Inflate a tire at sea level to 30 PSI as indicated by an internal pressure sensor. Now if one could take that tire to altitude, say 14,000 ft. (approx. height of Pikes Peak in the original question), what would the internal tire pressure be.
I say 30 PSI, because the tire construction does not allow growth of the tire casing, therefore the volumetric capacity of the tire has neither increased or decreased, therefore the internal pressure cannot increase or decrease.
However, as you correctly stated, the differential pressure between the inside tire pressure and the outside or atmospheric pressure would increase. This difference in pressure would be measureable with a standard tire gauge because it would be measuring the pressure difference. Therefore the pressure difference would be approx. 36 PSI because we have lost approx. 6 lbs of atmospheric pressure.
However, if the container were constructed in such a manner as to allow for uniform expansion of the internal gases, a balloon for instance, then the volumetric capacity would increase, and the internal pressure would actually decrease, (molecules would be further apart). In this last case, air mass is neither gained or lost but remains the same, therefore the internal pressure must then be reduced by the amount of expansion gained due to the loss of external atmospheric pressure. The container would expand until the internal and external differential pressures are equalized. Which in this particular case at 14,000 ft msl would be approx. 9 PSI or 17.70 in mercury (600 MB) compared to sea level pressure of approx. 15 psi or 29.92 in. mercury (1014 MB).
Yikes, I think I have a headache now.....

 

Z06vettepilot has just gotten an honorary Doctorate in my book. Every thing he said is sound engineering. The pressure in the tire does not increase, the delta pressure between the inside and outside does increase because the tire does not expand with increased pressure.

One more thing on tire pressure gages, they don't measure the differnece between ambient and measured pressure. The tire pressure gages we all use, including the ones in the tire, measure force against a spring. The only gage that measures against ambient pressure accurately is a delta pressure gage. The only one that is more affected by ambient pressure, but it is not linear, is a Bourdon Tube gage. Those need to be corrected daily against local ambient pressure

Do you concur Dr Z06vettepilot??

mk

 

Fantastic. But the differential pressure is what you need to be concerned with. Fill your tire to 30psi at sea level, then drive it up to a high altitude. If you're not correcting for altitude your internal pressure monitor will be deceiving-- your tires will still behave as if they are overinflated.

 

I'll have to think on that..... yes I think so.

Truthfully, I felt like Forest Gump after reading my own writing, I have no advanced degree.... but I did stay at a Holiday Inn Express last night.

 

Though the above discussion has gotten pretty technical, I think the easy answer for those of you living above sea level is to just set your tires to 30 psi on a regular tire gauge.

 

Yes I do agree that it is the differential pressure that is effecting the tire characteristics. But seeing that one cannot go much higher than Pikes Peak in an automobile and still recieve enough oxygen to continue driving, I guess it doesn't matter that much anyway, 6 - 7 lbs is about the greatest difference one could reasonably expect to encounter.
Actually, but I don't know this for certain, I would guess that the same system that constantly adjusts the fuel and spark delivery based on temperature and atmospheric pressure would also supply pressure information to the BCM that monitor's the tire pressure sensors also, that way the tire pressure readout would be accurate at any altitude.
But that is just a guess.

 

You might think so, but that would have required coordination of two different teams at GM.

 

I don't think coordination between teams is a required disipline at GM anymore...... probably why there are two different size wheels and tires from front to rear.
"Say, those 18" wheels look good back here"....
meanwhile up front.....
"I think 17" wheels will look just fine"...
"Ya think we should talk to the guys in the back?"
"What for?"

 

Go to this link:http://www.goodyear.com/truck/pdf/ra...tread_S5_V.pdf

read page 4