So a year ago I had my tires rotated at Costco in Florida at sea level as part of my normal ongoing service.
We then moved to 6000' elevation. Yesterday I had another rotation done at Costco at a store that is at 3000' elevation.
The service tech tells me that the tires had 50 PSI and that they lowered it to the recommended 35 for the vehicle.
I'm assuming the difference in elevations is the reason but I'm frankly kind of surprised at the amount of difference.
Does this seem reasonable from a physics standpoint or is something else at play?

All other factors being equal, you should see a *slight* increase in tire pressure-but not 15 psi difference.

So…something else may be at play.

Going up in elevation, the outside air pressure will decrease, causing the air in your tires to expand and the pressure to increase. But not by more then a few pounds. I'd have to look it up but I think it is less then 1 psi increase in pressure for every 1000 ft increase in elevation.

You can't go from 35 to 50 psi - a >40% increase - just from an altitude change. Somewhere between where you were and where you are, a human or mechanical error intervened. Also, whether 100% or 78% nitrogen (i.e., air) makes no difference.

quote:

Originally posted by 400m:
Going from sea level to a higher altitude your tire pressure should drop not rise. Temperature plays a factor too. When temp rises so does pressure, meaning a drop in temperature corresponds to a drop in pressure.

Sounds like something else is affecting your numbers.

Tire pressure should rise the higher you go.

Think about a balloon blown up 50' under water. What happens to the balloon as it floats up if you release it? it expands because there is a shorter column of water above it and therefore less pressure pressing down on it. Same applies the a rigid container except rather than expanding to meet the lower surrounding pressure, the pressure inside the rigid container increases.

The tire is the rigid container and it's a column of air instead of water above the tire that is is responsible for the pressure surrounding the tire. At sea level, there is literally 1 atmosphere of pressure outside the tire because the column of air above it is the thickness of the Earth's atmosphere.

To the OP, no something else is going on. If it was 40 degrees and they put 35psi in the tires in Florida and it was 90 degrees at 3,000' A YEAR later, might be 10psi higher. You need to check your tires more often than every year! I always check right after the shop; 8 out of 10 times they put too much in.

35-50psi would require going from sea level to space in order to totally remove atmospheric influence on gauge pressure. I'm pretty sure that didn't happen

Even if they were filled while stone cold and you drove them up a mountain as hard as you could and checked them hot…somebody’s pressure gauge is off lol

quote:

Think about a balloon blown up 50' under water. What happens to the balloon as it floats up if you release it? it expands because there is a shorter column of water above it and therefore less pressure pressing down on it. Same applies the a rigid container except rather than expanding to meet the lower surrounding pressure, the pressure inside the rigid container increases.

Someone set the tire pressures during an oil change during the last year, and likely over-inflated them. Also, if it was cold when they were set, and it is warm where you are, that could account for at least 10psi of that difference. I will see a pressure difference of 10psi+ on my autocross car over the course of a few laps if I don't release pressure in my tires. Its amazing how much they can fluctuate.

I see that you said that your tires are nitrogen-filled, but there is no standard for what that means. I'm in the industry, and a lot of places claiming "nitrogen" are 80% at best. That's not much more than regular old air. The whole nitrogen thing is a marketing gimmick for the most part.

quote:

Originally posted by trapper189:

quote:

Originally posted by 400m:
Going from sea level to a higher altitude your tire pressure should drop not rise. Temperature plays a factor too. When temp rises so does pressure, meaning a drop in temperature corresponds to a drop in pressure.

Sounds like something else is affecting your numbers.

Tire pressure should rise the higher you go.

Think about a balloon blown up 50' under water. What happens to the balloon as it floats up if you release it? it expands because there is a shorter column of water above it and therefore less pressure pressing down on it. Same applies the a rigid container except rather than expanding to meet the lower surrounding pressure, the pressure inside the rigid container increases.

The tire is the rigid container and it's a column of air instead of water above the tire that is is responsible for the pressure surrounding the tire. At sea level, there is literally 1 atmosphere of pressure outside the tire because the column of air above it is the thickness of the Earth's atmosphere.

To the OP, no something else is going on. If it was 40 degrees and they put 35psi in the tires in Florida and it was 90 degrees at 3,000' A YEAR later, might be 10psi higher. You need to check your tires more often than every year! I always check right after the shop; 8 out of 10 times they put too much in.

I'd agree there has to be operator error at the tire center end. Both were done at Costco after I had driven in for the service from an hour away each. And my understanding is that Nitrogen is less subject to hot/cold anyway.
I was really surprised when he told me it was 50 PSI.

quote:

I'd agree there has to be operator error at the tire center end.

Some - OK, many - of them look at the maximum pressure on the tire sidewall and think that's what it should be, not the recommended pressure on the car's door jamb label.

"Nitrogen-filled" ehh? Did they also top off the blinker fluid?

quote:

Originally posted by egregore:

quote:

I'd agree there has to be operator error at the tire center end.

Some - OK, many - of them look at the maximum pressure on the tire sidewall and think that's what it should be, not the recommended pressure on the car's door jamb label.

Ding Ding, I think we have the winner here! Florida Teen likely struck.

quote:

Originally posted by sgalczyn:
"Nitrogen-filled" ehh? Did they also top off the blinker fluid?

The blinker fluid was spot on. And as a bonus the muffler bearings still appeared properly lubricated.

quote:

And my understanding is that Nitrogen is less subject to hot/cold anyway.

Tire Rack: Should I Use Nitrogen In My Tires?

Excerpt:
"Nitrogen is a gas and is still affected by changes in ambient temperature (about one psi for every 10° Fahrenheit). Nitrogen filled tires will require pressure be added during the fall/winter months as ambient temperatures and tire pressures drop. Nitrogen is good but can't change the laws of physics."

I don't understand how pressure increases with elevation. If you set the pressure at "home" and then drive to a higher elevation, ignoring temperature influences, the pressure in the tire seems like it should remain constant. No air has entered or left the tire. The ambient pressure may have reduced, thus encouraging tire expansion. This expansion, with larger volume, may lead to decrease in tire pressure. But I don't understand how / why the tire pressure would increase with elevation.

quote:

Originally posted by Powers77:
And my understanding is that Nitrogen is less subject to hot/cold anyway.

Minimally less affected by temperature. For normal drivers it is a non-factor. Racing is a different story, with very different tires.

Average or typical temperature rise of the tire with regular air, and the concurrent pressure rise, when loaded and on the highway will be part of the calculation the vehicle manufacturer does to determine the recommended cold tire pressure. Starting out with the correct cold pressure in the morning with regular air in the tire is way safer than a nitrogen fill but ignoring the pressure after that.

I'm one of the weirdos that monitors my tire pressures regularly with a quality gauge. My wife and friends joke about it. I've never had a pressure rise anywhere near worrisome levels even loaded up in the summer.

Charlie Boyle's Law is at work in this situation.

Pressure A x Volume A / Temperature A = Pressure B x Volume B / Temperature B.

If temperature goes down (e.g. summer to winter), either pressure or volume must also go down. Since the tire volume (empty space) is a fixed constant value, the equation reduces to Pa / Ta = Pb / Tb. If Tb is less than Ta, then Pb must go down to compensate. We lose tire pressure at our altitude in the winter.

Of course, the tire pressure gauge is not absolute pressure, but gauge pressure (or gage pressure if you prefer). This means that P location A is actually atmospheric pressure + tire internal pressure (the pressure above the resistive surrounding air pressure).

So, the equation now reads...

(Pressure atmospheric A + Pressure tire A) x Tire Volume / Temperature A = (Pressure atmospheric B + Pressure tire B) x Tire Volume / Temperature B

If pressure atmospheric declines, tire volume remains the same, and temperature remains the same, we see the tire pressure rise to compensate (so to speak).

P sea level + P tire must equal P altitude (lower) + P tire (higher)

So, what is the change in pressure at 5000 feet? This is tabulated in many textbooks.

P sea level 14.7 psi + P tire 35 psi = P 5000 feet 12.2 psi + P tire ? psi

Solving for ? we see the tire pressure is 37.5 psi.

Obviously not 50 psi.

https://www.tirerack.com/upgra...dgndJXfwXUxgrmRHmd17

I go to a tire shop that uses a 78% nitrogen blend.

Unlike you peasants, I only fill my tires with a signature artisanal blend of nitrogen, oxygen, argon, carbon dioxide, and trace gases.

I hear it's only found on one planet in the entire solar system, and possibly the entire galaxy, so it's exceptionally rare.