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2019 Forester Limited
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Nope. The "OEM pressure" always refers to gauge pressure.

From the perspective of your tires, taking a properly inflated tire from LA to mile-high Denver where the atmospheric pressure pushing in on them is 2.5 PSI less, is no different than staying in LA and adding 2.5 PSI to your tires at the air pump. In either case, the tires are overinflated by 2.5 PSI.
 

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2011 Forester 2.5X Automatic
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170 Posts
A standard TPMS turns on the dash light when the tire pressure drops 25%(!) below the pressure the sensors are set at. So a typical 32 psi tire would have the light come on at a 24 psi.
I can pretty much assure you your neighbor probably hasn't a clue as to what the TPMS idiot light means nor will they take the time to pull out the book to find out. They'll just drive around that way with the light on. I've answered more than a few tire pressure questions and TPMS issues on other forums to know your average car owner knows squat about the vehicle they are driving and nothing of any warning indications. Ignorance is bliss ...'til it's not.
 

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2020 Forester Sport
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21 Posts
Discussion Starter #23
Nope. The "OEM pressure" always refers to gauge pressure.

From the perspective of your tires, taking a properly inflated tire from LA to mile-high Denver where the atmospheric pressure pushing in on them is 2.5 PSI less, is no different than staying in LA and adding 2.5 PSI to your tires at the air pump. In either case, the tires are overinflated by 2.5 PSI.
I see. I was thinking incorrectly that I needed to add air to make up for altitude. Since the TPMS shows the internal tire pressure, it would always read low at altitudes above sea level, unless there was a sensor for atmospheric pressure. I guess OEM specs are based on gauge readings because the external shape of the tires would change at altitude. So always using a gauge to set the tires to OEM PSI would keep tire shape the same regardless of altitude. Thanks for the info.
 

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2019 Touring
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218 Posts
Trust but verify - My Subaru at sea level reads pretty accurately. Within a pound. I have the same on my motorcycle and it too is accurate. It's interesting to watch the pressure as the tires heat up during use, but if it's set correctly it will give you good information.

My PITA complaint is when my tail gate will not open and I have to disconnect my battery for a minute. The my tire pressure number reverts to metric ! It's a pain to reset... You would think that if the speedometer is english, everything would be the same. Why not ???????
 

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2020 Forester Sport
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Discussion Starter #27
Trust but verify - My Subaru at sea level reads pretty accurately. Within a pound. I have the same on my motorcycle and it too is accurate. It's interesting to watch the pressure as the tires heat up during use, but if it's set correctly it will give you good information.

My PITA complaint is when my tail gate will not open and I have to disconnect my battery for a minute. The my tire pressure number reverts to metric ! It's a pain to reset... You would think that if the speedometer is english, everything would be the same. Why not ???????
I haven't had that tail gate problem on my 2020. Is that a common problem? Hopefully they've resolved that issue.
 

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2019 Touring CVT
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R
Trust but verify - My Subaru at sea level reads pretty accurately. Within a pound. I have the same on my motorcycle and it too is accurate. It's interesting to watch the pressure as the tires heat up during use, but if it's set correctly it will give you good information.

My PITA complaint is when my tail gate will not open and I have to disconnect my battery for a minute. The my tire pressure number reverts to metric ! It's a pain to reset... You would think that if the speedometer is english, everything would be the same. Why not ???????
Rather than disconnecting the battery when the gate gives you 3 beeps and fails to open, press and hold the button above the license plate until the gate releases. This is a known solution.
 

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2019 Touring
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Pressing and holding the button above the license plate did not work. That was the way I initially tried to open it, then I tried the button inside, near the drivers door. that too, did not work.
 

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2019 Forester Limited
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Maybe your VIN is among those mentioned in a TSB as needing a replacement rear gate controller.
 

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2019 Forester - Touring CVT
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36 Posts
IMHO, this is simply more needless "technological" **** car makers ...including Subaru ...are foisting onto the "technological cognoscenti" ...
This is incorrect. As with most of these safety improvements, they are mandated by the Government, not the auto maker. TPMS became federal law for all passenger vehicles produced after September 2007. This was driven by the Firestone tire under-inflation & roll over issue that Ford had.

Auto Stop/Start is another example. If a vehicle is EPA certified with Auto Stop/Start, then the vehicle design must not allow the owner to permanently defeat Auto Start/Stop. It can only be temporarily disabled by the owner for the current drive cycle.
 

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The Nanny State lives.

The reason the tires failed on the Ford Explorer (the old full-size version I believe it was) had to do with the ridiculously low inflation pressures Ford recommended on this vehicle ...the more to make, essentially, a truck-based SUV ride like a car than the truck it's based on. You could've put any tires on that Ford truck SUV and, with the low inflation pressures, have the tire fail catastrophically because so many owners don't take the personal responsibility to check tire pressures ...ever.

And the only reason for the Stop/Start systems was to boost the city EPA mileage ratings on gas piggy vehicles such as full size SUV's and the like. This has nothing to do with safety nor the Government mandating anything; the automakers came up with this stupid system for their own good all cached and marketed in the "green" trying to save the planet mentality. But because the makers certified the car with this ridiculous system it has to default to function all the time. In some vehicles ...like a Subaru as I understand it ...you the owner can't turn it off, ever.
 
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I am no spring chicken and I appreciate having TPMS. I am AR about exact tire pressure especially in winter so as not to have to deal with the cold and mess and summer mainly for for handling reasons. My other vehicle shows exactly the same reading as my ryobi inflator digital readout says.I will check my 2020 Forester once I read the manual and figure out how to.
 
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My understanding is that Ford wanted to save money and chose a 15" tire instead of a 16" that Firestone recommended knowing that Explorer owners are not typically vigilant about TP and also tend to overload the vehicles. Under sized tires, overloaded vehicles, under inflation and southern climes all came together to cause excess heat and thus blowouts and or tread separation.
 

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2012 Forester X Auto
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@denissh -
Really...
"TPMS is fixed to Sea Level ... most external gauges display based on your Altitude...
at 10000ft is like 4.6 psi difference"
TPMS is fixed to sea level?
:ROFLMAO:
TPMS isn't "fixed" to anything except the wheel it is attached to.
Just like any pressure gauge TPMS reads the current pressure of the tire.. It is a pressure gauge transmitter..
When the outside atmospheric pressure decreases due to driving to a significantly higher elevation, the measured internal pressure of the tire by either TPMS or a standard pressure gauge will increase, because tire pressure is a differential between the squeeze of the atmosphere and the internal air in the tire pressing against it.
The same thing happens in a balloon - They expand as they rise due to the atmospheric pressure decrease.

The inverse happens when you set your cold tire pressure at high elevation and drive down to (or below) sea level.
The pressure reading will go down, because the tire is squeezed harder by the atmosphere.

In either case, it is appropriate to check and adjust cold tire pressure for the conditions that you are in, and in addition to altitude, temperature has a considerable influence, which is why so many people have problems with underinflated tires when summer heat becomes winter cold and tire pressure drops.

Sea level air pressure has nothing to do with any pressure reading, nor is TPMS "aware" of that, nor should it be, as it would report pressure in the tires to be either under or over inflated for the tire's current elevation if not at sea level.
BTW - sea level pressure also changes

To try to explain a TPMS vs pressure gauge reading difference due to the "fixed to Sea Level" might be creative, but it is completely wrong.
 

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2019 Forester Limited
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It's not an unknown problem. Hondas have (had?) a procedure to recalibrate for the altitude effect @denissh speaks to.

The pressure increase seen with a standard gauge (gauge pressure) after ascending is due to the sensor (a bourdon tube, for example) being exposed to internal tire pressure (absolute pressure) on one side and atmospheric pressure on the other. Gauge pressure = absolute pressure - atmospheric pressure. This is the differential pressure mentioned in your post. Only the atmospheric pressure changes while ascending, while absolute pressure remains constant (i.e. tire expansion is negligible).

The question at hand is, does a TPMS read absolute or gauge pressure? If a TPMS and a standard gauge are to give identical readouts under all situations, as you say they do, then the TPMS MUST display gauge pressure. To do that, the TPMS sensor MUST somehow see atmospheric pressure, as well as internal tire pressure, just like a standard gauge. How does it do that? Your post doesn't explain. I suppose the TPMS module (in the car, not the tire) could have a sensor chip reading atmospheric pressure, but that's a guess. Or, as @denissh suggests, 14.7 PSI is subtracted from the TPMS readout to obtain a sea level gauge pressure (and erroneous TPMS readings at altitude).

Including temperature effects, and comparisons to expanding balloons in the discussion are not helpful. Assume constant temperature, and negligible tire expansion. Unlike baloons, tires are constrained.

The pressure reading will go down, because the tire is squeezed harder by the atmosphere.
The standard gauge pressure reading goes down because atmospheric pressure has increased (gauge pressure = absolute - atmospheric). "Squeezing" the tire would only impact internal pressure if the tire's volume shrank, and in that case the absolute pressure would increase slightly, not decrease. Best to neglect minor effects such as this.
 

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Although a tire is relatively solid, it is not immune from pressure effects, like a steel tank would be.
It can expand and contract like a balloon does, although obviously nowhere near to the same extent.
This quote I thought was a decent explanation... Oops - Lost the link...
"
ALTITUDE
Changes in the altitude that a car is used in have an effect on the tire pressure because the external air pressure has changed. The level of pressure in a tire is created from the relationship between the air inside the tire and the air outside. As you climb to a higher altitude, the air offers less resistance, which will create a larger amount of pressure within the tire. Accordingly, it is necessary to make adjustments for a change in altitude; releasing air for ascent and adding air for descent. Tire pressure and altitude go hand in hand because the qualities of the air outside differ according to how high you are above sea level."

That said, any difference is pretty small, and like you said, the difference in the tire gauge reading due to (mostly) a reduction in atmospheric pressure with altitude accounts for most of the observed variation, as nearly all pressure gauges are "designed with differential in mind", and although absolute pressure gauges are available, these are not the norm, nor are they needed for tire pressure assessment.

In any case, an interesting discussion.
For most people, it is probably much more important to address tire pressure with the temperature factor in mind.
As it gets cold, if tires are not set to a proper pressure at the ambient low temperature, they will be under inflated.

Last point - Relying on the "standard" TPMS warning light for monitoring tire pressure is a generally bad idea.

It was originally implemented to address the Ford Exploder tire failure and rollovers, not as a system to ensure optimal tire pressure. Therefore, the warning light only indicates a dangerous under inflated condition.
Long before the warning is issued, your mpg's have been reduced, tire wear has been increased and proper handling of the car has been compromised. The newer models provide more information as to specific tire pressure, but what they won't tell you is if there is any tire damage (like a side wall bubble that could cause a blow out) or if there is abnormal wear that you would see with an "old fashioned" pressure check..
 

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2019 Forester Touring CVT7
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wow big post after 5 month ...
TPMS read (vacuum - 14.7 psi) set starting point (=sea level ), it will not care for atmospheric pressure as it is inside the tire,
your external gauge(most) reads relative to local atmospheric pressure.

2019+ OEM Tire is 51PSI max (cold) @ 1,653 lbs per tire over 6000lbs total
so if you car tell you its 35 and your gauge showing 40 you still be fine

so it is not needed to keep adjusting your tire pressure every time you drive uphill ...
if you menage to send your car to space you still be under max tire pressure 35+14.7 = 49.7 ..
 

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While true, that doesn't mean you have proper pressure in your tire.
The other thing is are you sure that your reading from TPMS is accurate?
Since many people notice their tires only when flat, any usage of a measurement system, and an attempt to keep them properly inflated is a big step up from neglect.
 

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2019 Forester Limited
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Answering the original question, NXP quotes an accuracy of +- 5 kPa (0.72 PSI) for their automotove TPMS chips. That’s the chip only.

A decent hand pressure gauge is +-0.5 psi, so you can easily have a true pressure of 35.5, that displays as 36 psi in the car, and 34 psi with your hand gauge. Close ‘nuff.
 
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