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Discussion Starter · #1 ·
I believe I've read all of the info on TPMS and have the following simple (well not really) question. I live at high altitute in Colorado and when I fill the tires (using the front tires for example) to 32 psi per my guage (I have tried many different types of guages), the TPMS reads about 4-5psi lower.

From what I understand about guages and TPMS, this might be because the guage is reading the difference between the outside pressure and the tire pressure while the TPMS has no idea of the atmospheric pressure. Which to me would make the pressure on the guage higher due to lower base atmospheric pressure as a benchmark up here. OK, hopefully I'm not off base with all of my assumptions yet regardless, what I am supposed to do, either the guage reads WAY high or the TPMS reads way low???

Please help!
 

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Hi Skirbs,
If my envelope scratching is right - your situation could be fairly normal (while, at first, I thought it sounded backwards too).

If the handheld gauge is referencing to (zero-ed at) a lower atmospheric pressure (lets say 12 PSIA instead of the typical 14.7 PSIA at sea level), and one then inflates the tire to add 32 PSI above this, the absolute pressure would then be 44 PSIA.

Assuming the TPMS sensors were calibrated to be zero at sea level - when presented with 44 PSIA, they would then subtract 14.7 PSI and display 29.3 PSIG.

The above numbers are off - but the concept is the same - the TPMS could always read lower than the hand-held gauge (depending on what each references as zero).

My guess is the local Porsche dealer has dealt with this for their entire existence and will have age-old recommendations/policies.

Good luck.
 

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Discussion Starter · #3 ·
I called the local dealership and they claimed to ignore TPMS all together and only rely on it as a warning system for a flat tire. According to TireRack.com, one should also rely on the guage (I think I'm understanding their article correctly), because as we gain altitude the TPMS is less accurate as shown in the chart below. Any other experiences/opinions with this?

From TireRack.com:

Significant changes in altitude affects tire pressures when traveling from one elevation to another. Fortunately this influence is relatively small and can be easily accommodated.

Atmospheric pressure is the force exerted on objects by the weight of the air molecules above them. While air molecules are invisible, they have mass and occupy space.

However as altitude increases, atmospheric pressure decreases. For example, atmospheric pressure pushes against the earth at 14.7 pounds per square inch (1 kilogram per square centimeter) at sea level, yet drops to only 10.1 pounds per square inch at 10,000 feet as indicated in the following chart.

Altitude (ft.) Air Pressure (psi)
Sea Level 14.7
1,000 14.2
2,000 13.7
3,000 13.2
4,000 12.7
5,000 12.2
6,000 11.7
7,000 11.3
8,000 10.9
9,000 10.5
10,000 10.1
When it comes to measuring tire inflation pressure, it is important to realize there is a difference between atmospheric pressure and gauge pressure. Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure.

Imagine removing the core from a tire valve and allowing the air to escape. Even after the air has completely stopped rushing out of the valve, the tire is still experiencing 14.7 pounds per square inch of atmospheric pressure. However, a tire pressure gauge would read zero pounds per square inch of tire inflation pressure because the pressure outside the tire is equal to the pressure inside.

Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.

However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10-degree Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations.

This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home.
 

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i'm at sea level and the tpms always reads 1 to 2 psi lower than my longacre gauge. if my gauge reads 32psi on the front tires, tpms will frequently read 30psi.
 

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Think the tire as a "sealed" container. The pressure inside this "container" should be independ from the relative pressure so these four "containers /tires" could afford the weight of the vehicle and performing as it should be. The normal pressure gauge we are using is "outside" the tires. In your area, it's better to follow the "absolute pressure".
The article you read is right that the differences will not cause TPMS warning light poped out, but a lower tire pressure will definately burn more gas.
 

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Discussion Starter · #8 ·
Jimbo,

So are you saying to follow the TPMS because that is in opposition of what I understood the article and others to be saying. The way I was told to think about it was if you inflate a baloon to it's optimal size at sea level then bring it to my house, although the pressure in the baloon has remained the same, it's now far bigger and beyond it's optimal size, it would need to be deflated a bit to regain it's correct shape, thus a guage measuring it's relative inflation to the outside pressure would be required.

Please help me understand.
 

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I've never lived at high elevation, and so am not familiar with the time-honored accepted practices. But, looking at this from the outside (and not Google searching, etc) my guess would be that the absolute pressure inside the tire is the bottom line (as Jimbo says above). I wouldn't be as concerned with tire shape, as my guess is it doesn't change much for these pressure differences. Tire 'spring rate' however will vary according to the absolute pressure in the tire.

Long story short, my guess is you should rely more on a good handheld gauge and add a little (gauge) pressure to compensate for the lack of atmosperic pressure, to achieve the intended absolute pressure.

For example, if you live at 5000 feet, then you could add 2.5 PSIG.

I would also pay attention to tire wear patterns (if the center wears out first, pressure was too high; if both sides (inside and outside) wear out first, pressure was too low).

I totally expected some PhD physicist/professor type would chime in and overshadow my guesses. Until then, those are my guesses. Good luck.
 

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As others have said use the TPMS for a warning system and a tire guage to set the actual pressure. Discounting side wall stiffness a tire does it's job of supporting the cars weight by the difference in pressure from inside to outside over the area of the contact patch. The math is simple, weight = guage pressure X contact patch area.

For a given pressure the patch will change in size a bit as you add or subtract the weight on it. Either by adding payload or by weight transfer when turning. This is also why an underiflated tire spreads out more to carry the same weight. Harder to see with a radial but it does change.

If one were to use absolute pressure for filling the tire without taking the ambient pressure into account the tire could be under or over inflated for the conditions, just as you observed with the balloon. It would all depend on what conditions the absolute pressure was intended for.

The one thing to pay attention to is significant altitude changes. If you set you tire to 30 psig (guage pressure) at 6000 ft and come down to sea level you'd find that tires would be now at roughly 27 psig. It would work the opposite direction if you were to drive up to 12,000 feet and be around 33 psig. So if you drive down to San Diego for the weekend you'd want to adjust the tire pressure but beyond that not a big worry.

This relationship holds for altitudes we can breath at. If you go high enough the lapse rate for ambient pressure changes but for tire pressures where we drive it's close enough.
 

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I checked my pressures today both cold and hot (after a good drive) and my tpms reads 4 psi lower on all four corners than my trusted guage. I live at 4500 ft and the temp this morning was 45 degrees.
 

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I live at sea level and my TPMS read 4 PSI lower than a tire gauge.
 

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I'm at 708 feet and must be one of the lucky one. TPMS is only 1 to 1.5 psi low compared to a couple different guages.

There was a pole on this a few months back for the variation. Only a bunch of German engineers could take a useful warning device and make it complicated...
 

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All the concerns and discussions are correct. The point is the way ECU monitors tire pressure differs from pressure gauges we have been using.
ECU receives pressure reading from sensors in the tire which is not affected by the outside ambient except for the temperature inside tires, and ignore every factor that is below "0". Easier way to test is what will happen if we put a tire in the water under the sea level? If all other conditions are the same, for instance, the temperature. The pressure inside the tire is the same no matter it's in CO. or Dead Valley, but the gauge reading changes due to the elevation. Number of molecules in the tire is fixed if it's sealed. Nothing is wrong, we just need to use the same way as the ECU does so we won't be confused by the differences.
 

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So I guess the question is what to do?

Simple engineering problem...
The tyres should be set via the gauge relative to ambient air pressure. This will ensure that the tyre shape and characteristics remain the same.
Using an external gauge it is effectively zeroed or calibrated to the ambient air pressure. The 32 PSI is a relative pressure measurement based at sea level.
Eg, As an extreme case, say you lived on the moon then you would inflate your tyres to 17.3PSI. 17.3=32-14.7(air pressure at sea level) PSI. If you did not do this then the tryes would start to take on the shape of a Donut.

TMPS should only be used as a guide. Accuracy could be improved if this system can be recalibrated.
 

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I live in Colorado at 6500 ft.....this weekend I checked the tires after having driven it and my hand held guage said 34 front & 36 Back which is exactly what my TPMS indicated. I have a Gen II so I don't know if they improved the accuracy or I am just lucky tp have one that it accurate!
 

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Can anyone else at high altitute confirm that their TPMS reads low consistantly yet you rely on hand-guage?
I wrote one of the old posts here on P9 regarding TPMS and the flaw of using absolute pressure sensors. A better design would include a 5th pressure sensor to measure ambient pressure and report the difference between readings inside each tire and a common/reference ambient pressure sensor. I live about an hour north of Denver, and yes, the TPMS always reads low by ~3 PSI at 5000 feet above sea level. Use (and trust) a high quality tire pressure gauge. If you want to be sure the TPMS doesn't ever give you false low pressure warnings, run just 1-2 PSI above the recommended pressures. I have found that running my tires exactly at the recommended pressures will result in occational TPMS false warnings about a low tire. It happens most frequently when the sun angle just happens to heat up the value and TPMS unit, but no the whole tire.

The advice the dealer gave you to use TPMS solely as a warning for low tires is spot on!

Don't forget that the TPMS readings and warnings are scaled as a function of tire temperature too. There's even a graph in the owners manual covering this. You will find that TPMS vs. gauge reading offsets will differ strongly as a function of temperature. If I go out on the track with my street tires TPMS will ALWAYS give warnings - initially because my cold starting pressures are indeed too low and after a few laps because the temperature compensation expects a much higher pressure than I actually have. The moral of this story is don't waste money on TPMS sensors for track wheels!
 

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The TPMS indicates deviations between the tires pressure after a baseline is set - at whatever pressure that is. It is not related to exterior temps or pressures only deltas between the 4 corners corners.
 
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