Elevations' effect on Tire Pressure

[chesspirate]

I started thinking abou this due to a mini science experiment taking place in my car almost every day. I drink from a water bottle I refill regularly, and quite often in the morning when i get in the car, it's empty and by the time i get to work, it is sucked in and out of shape, due to pressure changes from elevation differences.

I live at about 4000ft above sea level and work below 1000ft above sea level. So, my question, since i'm having a hard time thinking this through myself, is at what pressure will i be given a certain pressure at a certain elevation???

As it stands now, i'm at 38psi all around in the morning, cold at 4000ft.

So, is that really to low? I guess I could check the pressure after work before going home, but that would be on a different machine, I guess I'll go out and buy another tire pressure guage!

[ZugyNA]

I do know that if you live at around 1000 ft and inflate your tires to the recommended pressure...and then drive up into a pass in Colorado...there's a good chance your tires are overinflated and might get cut on a sharp rock.

Not sure how to do the math to determine altitude relative pressure.

[krousdb]

Originally Posted by ZugyNA

I do know that if you live at around 1000 ft and inflate your tires to the recommended pressure...and then drive up into a pass in Colorado...there's a good chance your tires are overinflated and might get cut on a sharp rock.

Not sure how to do the math to determine altitude relative pressure.

How do you know that? What does overinflation have to do with your tire getting cut with a sharp rock?

I'm not the sharpest tool in the shed, but I wouldn't think that pressure would increase with elevation. The thermo equation PV/T=PV/T would apply. Assuming that temperature and tire volume are constant at both low and high altitudes, you are left with P=P, or no change in pressure. But if you assume that the volume of the tire increases due to lower atmospheric pressure (the difference in pressure inside vs outside the tire increases so the tire might expand) then the tire pressure would actully go down.

But what do I know, I suck mightily.

[Sludgy]

Originally Posted by ZugyNA

I do know that if you live at around 1000 ft and inflate your tires to the recommended pressure...and then drive up into a pass in Colorado...there's a good chance your tires are overinflated and might get cut on a sharp rock.

Not sure how to do the math to determine altitude relative pressure.

A tire inflated to "38 psi" is actually inflated to 38 "psig" (pounds per square inch, gage). A sea level, this same pressure is 14.7+38 "psia" (pounds per square inch, absolute). Sea level air pressure is 14.7.

Tire psia doesn't change with altitude, but psig does. At 1000 feet elevation air pressure is ~14.3. The tire psig would be 38 + 14.7 - 14.3 = 38.4.

At 5000 feet the tire psig would be about 38 + 14.7 - 12.5 = 40.2.

Note that the above calcs are for CONSTANT TEMPERATURE. In actual fact, the temperature decreases with altitude. The temperature effect is much smaller than the altitude effect.

[chesspirate]

thanks sludgy,

that was what i was thinking, that at higher elevations the number (however you want to call it) would be higher, due to the atmospheric pressure

[chesspirate]

Okay, so like it says in my Gaslog, i'm now up to 40psi in the morning, at home at around 4,000feet.

The other day, i went to the dealer for the 5,000 checkup or whatever, and that included tire rotation... Got home and checked on them... 31psi... so i called the dealer and asked "what thier policy was on tire pressure" and the service lady said "32-34psi for better ride, handling, and tire life"

Max psi on sidewall is 44psi...

I've heard all the arguments about higher pressure leading to increased wear, but what do those that push the envelope of pressure think?

And i thought it was funny she didn't mention mileage in her little blurb (that shounded like it had been drilled into her), i almost thought about asking, but then thought better of testing a dealer service person.

[SVOboy]

I don't see that it affects tire wear at all. Unless they forgot to put the steel belts in your tires.

[ZugyNA]

Originally Posted by chesspirate

I've heard all the arguments about higher pressure leading to increased wear, but what do those that push the envelope of pressure think?

What most people might not realize is that the typical OEM radial tire is usually an inexpensive tire that gives good mileage. If you check some of the tirerack.com reviews...a lot of people can't wait to get them off of their car.

Aftermarket tires can be made in a different way and can require higher pressures.

You really need to try higher pressures than what's on the sticker with some aftermarket tires....though some tires can LOSE traction if overinflated.

One method is to drive the tire over a wet patch on pavement or use chalk or whatever to see if the full tread is hitting the road.

I run my tires at their max pressure...I might have somewhat less traction...but I have better handling.