Good article on tire pressure

[mswlogo]

The gist of the article is NEVER under inflate. Even by 0.5 psi.

But it’s perfectly safe to over inflate some up to the max pressure on the sidewall.

What’s the acceptable range for over- and under-inflation of tires?

 

[Ostrichsak]

I just wish Tesla would add an algorithm to the cars to factor elevation on the TPMS reading. The car knows where it is so this should be simple enough. Where we live at elevation it's off by 3psi and that's even worse at higher altitudes. Couple that with the temperature fluctuations and you have a recipe for a needlessly dance that happens regularly. Tesla could easily take that reading and adjust it for the elevation the car is at to display a more accurate reading on the dash.

 

[meomyo]

I just wish Tesla would add an algorithm to the cars to factor elevation on the TPMS reading. The car knows where it is so this should be simple enough. Where we live at elevation it's off by 3psi and that's even worse at higher altitudes. Couple that with the temperature fluctuations and you have a recipe for a needlessly dance that happens regularly. Tesla could easily take that reading and adjust it for the elevation the car is at to display a more accurate reading on the dash.

i agree being in colorado as well....its a total dance. My gf freaks out when it gets low and i try to tell her not to worry then it goes up when she drives alot...i stay at 42 or so

 

[Ostrichsak]

i agree being in colorado as well....its a total dance. My gf freaks out when it gets low and i try to tell her not to worry then it goes up when she drives alot...i stay at 42 or so

42/43 is what we target as well. This is a difficult enough dance given our temperature fluctuations and it's affects on tire pressure. 81 degrees one day then 20's the next. It's made even more difficult given we have roughly a 3psi shorter window until we start getting warning messages and the car starts freaking out. Given Tesla has put these TPMS in cars 7+ years now I'm amazed they haven't already done something about it given how easy of an add this would be.

 

[mswlogo]

42/43 is what we target as well. This is a difficult enough dance given our temperature fluctuations and it's affects on tire pressure. 81 degrees one day then 20's the next. It's made even more difficult given we have roughly a 3psi shorter window until we start getting warning messages and the car starts freaking out. Given Tesla has put these TPMS in cars 7+ years now I'm amazed they haven't already done something about it given how easy of an add this would be.

When you say “target” do you mean you want to be at 42/43 (while your driving at temp and elevation) or just elevation.

I wish Tesla could show the ideal “target” they are shooting for. It’s obviously not 42.

One thing I think I’m learning is. Some tires warm up more than others (due to different rolling resistance) in the same conditions. And that is accounting for some of the efficiency differences I see in tires.

 

[Ostrichsak]

When you say “target” do you mean you want to be at 42/43 (while your driving at temp and elevation) or just elevation.

I wish Tesla could show the ideal “target” they are shooting for. It’s obviously not 42.

One thing I think I’m learning is. Some tires warm up more than others (due to different rolling resistance) in the same conditions. And that is accounting for some of the efficiency differences I see in tires.

It's important to understand the basic fact that the other poster and I are talking about: Elevation directly affects the displayed pressure versus what the pressure is in the tires at any temperature.

So, when we say we "target" 42psi that's just what the car says the tire pressure is. At ~5,000ft above sea level that's right at 45psi measured with a device that's calibrated for this elevation. When my tires are cold and I first start driving I'm lookin for 42psi at this elevation because 45psi displayed is actually 48psi in the tires. (45psi + 3psi elevation adjustment)

That means that 42psi actual will also trigger the car's warnings at 39psi since that's what the car thinks the tire pressure is. In reality, the cars are warning those of us at elevation when our tires are still at 42psi. For those at sea-level, imagine if your car started freaking out every time your tires were showing 42psi to get a sense of what we experience at elevation. Now, for added fun, add in wild temperature swings that we also experience at this elevation which also wreaks havoc on tire pressure and you'll start to get a sense of what we're talking about.

I know that wasn't the topic of this thread and we've ventured off topic but it's related and I'm still not sure why Tesla hasn't ever addressed this.

 

[303JeffB]

It's important to understand the basic fact that the other poster and I are talking about: Elevation directly affects the displayed pressure versus what the pressure is in the tires at any temperature.

As long as we're already off topic, I'd like to make a little clarification.

"Elevation change directly affects the displayed pressure versus what the pressure is in the tires at any temperature."

The pressure measured with a pressure gauge or the TPMS sensor is referred to as psig (gauge pressure) and is the actual pressure inside the tire and is not affected by local atmospheric pressure. Absolute pressure (psia) is the sum of atmospheric pressure and gauge pressure.

Think of a tire that has been fully deflated at sea level. In this condition, the tire will register 0 psig, but will be filled with air at atmospheric pressure (14.7 psi). When you then inflate the tire to 45 psig, the absolute pressure in the tire will be 45 + 14.7 = 59.7 psia. The number of air molecules in the tire is now established and fixed. Now drive the car to 5000' elevation and atmospheric pressure now drops to 12.2 psi.

We can use the ideal gas law, PV=nRT to understand how the pressure changes with elevation changes.

P = Pressure in psia
V = volume (fixed in this case)
n = Number of molecules (fixed in this case)
R = a constant to make all the units work
T = Absolute Temperature (also fixed)

Since the Volume of the tire doesn't change, the number of molecules don't change and we have waited until the temperature has returned to the original temperature, the Absolute Pressure won't change either.

Absolute pressure is the sum of atmospheric pressure and gauge pressure, so by climbing 5000' in elevation, gauge pressure in the tire increases to
59.7 -12.2 = 47.5 psig.

It works the same way going downhill:

deflate tire at 5000', atmospheric pressure = 12.2 psi
inflate to 45 psig
Absolute pressure is 57.2 psia

Drive back to sea level
Absolute pressure = 57.2 psia
Atmospheric Pressure = 14.7 psi
Gauge pressure = 57.2 psia - 14.7 psi = 42.5 psig

 

[Barry]

Elevation change directly affects the displayed pressure

I don't believe this is quite correct.

When you change tires or recalibrate/reset the TPMS system, that's the reference elevation that's used until the next reset of the system.

For example, as already stated, here in Denver, the TPMS reads 3 PSI low. If you drive to sea level, it will still read 3 psi low. If you recalibrate/reset the TPMS while at sea level, when you return to Colorado the TPMS will read the actual pressure, as referenced to sea level.

 

[303JeffB]

I don't believe this is quite correct.

When you change tires or recalibrate/reset the TPMS system, that's the reference elevation that's used until the next reset of the system.

For example, as already stated, here in Denver, the TPMS reads 3 PSI low. If you drive to sea level, it will still read 3 psi low. If you recalibrate/reset the TPMS while at sea level, when you return to Colorado the TPMS will read the actual pressure, as referenced to sea level.

I don't understand why the behavior you've described would be "programmed" into the system.


First (and most importantly) - Tesla recommends 45 psig in the 19" tires on my Model S. If you inflate the tires to 45 psig, why would they make the TPMS sensors display an adjusted pressure that is different than the actual pressure in the tire? As I tried to describe, the gauge pressure in the tire is what you measure with a gauge, as long as you remain at the same elevation, this pressure is entirely independent of the atmospheric pressure on the outside of the tire.

Second - Examining your -3 psi example, if that -3 psi was applied to the final pressure at sea level, then the displayed pressure should be almost 6 psi lower (see the math in my previous post).

Starting pressure = 45 psig
Reference Adjustment = -3 psi
Actual Atmospheric pressure change = -2.5 psi.
Predicted Display pressure = 39.5 psig

Third - if there's a 0 psi adjustment for elevation at sea level, when you return to Colorado, you should see a 2.5 psi increase in the tire pressure.

Finally - I made a drive this week from Denver to Las Vegas, elevation 2000'. I set the tire pressure at 45 psig the evening before I left, starting with cold tires. The displayed pressure in the car started out between 42.5 and 43 psi, within 2 blocks the car had identified the sensors and had reset to 45 psi.

But the difference here is that the temperatures earlier this week were 105 daytime, 80 nighttime and the roads and parking lots are hotter than that. Each 10 degrees will increase tire pressure by about 1 psi, and I'd estimate the tires were seeing at least 20 degrees higher temperature than in m y garage at home.

The tires are reading 45.5 - 46 psi right now, which is very close to what the math above would indicate (with temperature impact)

Starting pressure = 45 psig
Actual Atmospheric pressure change = -1.5 psi
Temperature variance = +2 psi
Predicted Display pressure = 45.5 psig


I understand you are seeing something different, but I don't think an elevation adjustment embedded in the system is the answer.

 

[Barry]

You make some good points, but I speak from real-world experience.

When I bought the car (new), the TPMS read correctly. Presumably, the tires were installed and the TPMS calibrated in Fremont. It continued to read correctly until my first tire rotation in Denver, at which time the 3 psi difference occurred.

 

[zoomer0056]

I just wish Tesla would add an algorithm to the cars to factor elevation on the TPMS reading. The car knows where it is so this should be simple enough. Where we live at elevation it's off by 3psi and that's even worse at higher altitudes. Couple that with the temperature fluctuations and you have a recipe for a needlessly dance that happens regularly. Tesla could easily take that reading and adjust it for the elevation the car is at to display a more accurate reading on the dash.

This doesn't make sense to me. I think the pressure readings are temperature and elevation agnostic. The pressure is of course affected by these but the sensor only shows current pressures no matter what the conditions are.

 

[Ostrichsak]

There appears to be lots of confusion going on here regarding several different elevation/air pressure related events.

When TPMS (and other pressure metering devices) are manufactured, they are calibrated for sea-level. As these are taken up in elevation, without the ability to recalibrate, they are inherently off. At the elevation I live at (~5,000ft) this equates to almost exactly 3psi. That is to say the car reads them as 42psi even though the actual pressure of the tires is 45psi if you meter them using a device calibrated for elevation.

This means that we're in danger of trigger the low pressure warning at a very nominal 43psi. Keep in mind that this is AFTER setting proper tires pressure based on devices calibrated to this elevation. The TPMS will display 3psi lower than what the tires are actually at. This will then vary even further if temperature changes or I drive up or down in elevation so I know what others are mentioning and I'm not disagreeing... I'm saying this is different and EVERYONE at higher elevation experiences it, whether they notice or not. I just happen to have my tires PSI in the display at all times so I'm more aware of what my pressure is when driving than the average driver.

I would be surprised if people here were driving around at that pressure regularly without thinking anything of it. Imagine if you were getting warnings all the time at this pressure level.

If I go up higher in elevation, this discrepancy increases even further and can read nearly 5psi of difference in some mountain towns we frequent. This means that even though your tires are actually at 45psi, the TPMS system in the car could actually read below 40psi which triggers the warnings even if you adjust pressure while you're up there using devices properly calibrated to that elevation. This is independent of temperature changes and the difference of the pressure as you increase or decrease elevation w/o correcting pressure which are different things that occur related to pressure and elevation.

Another example is a Dewalt inflator I own. It will read 3psi off every time I power it on because the internal default pressure metering device inside is calibrated for sea-level. There is a sequence you can do each time you power it on that causes the inflator to recalibrate for the elevation which artificially compensates for this. It artificially adjusts the reading to compensate for the difference in pressure at the given elevation. The crappy part is that it doesn't store this new calibration which means that I must do this each time I power it on if I want my pressure to be accurate.

 

[Barry]

If I go up higher in elevation, this discrepancy increases even further and can read nearly 5psi of difference in some mountain towns we frequent.

I have not experienced this. Mine maintains its 3 psi error because it was last recalibrated last tire rotation in the Denver metro. It only should show a 5 psi error if you recalibrate in the mountains.

 

[Art VandeIay]

Quite interesting article/thread. Here's my personal experience:

My wife and I do a Costco run every week (Not my cup of tea, but my wife loves going to Costco, so happy wife happy life!), and we either take my Model S or her Model X. (95% of the time we take her Model X). So every 2 weeks or so our routine changes to accommodate a tire check-up procedure.
Once we get there, I leave her at the store's entrance and I drive a few feet towards the tire center in order to use their complementary nitrogen filling stations:

According to our Model X' Tire and Loading Information label, our 20" slipstreams should be at 42 psi cold. We live at 8,622 feet of elevation, so that's around 4 psi of difference from sea level. We live quite close to our nearest Costco, so we just travel 3 miles from home on pure local roads, so not much time/distance/speed for tires to warm up.

So now I park at the nitrogen filling station and I set the machine to 46 psi. Once I fill all 4 tires, I move our Model X to the parking lot and join my wife inside the store. We usually take around an hour to do our shopping, so this gives the tires more than enough time to reach ambient temperature, although they are filled with nitrogen and they don't actually have a long enough trip to heat up as much.
Once out, I pack all the things in the trunk and then procede to measure the tire pressures again, but now using this Michelin Digital Tire Pressure Gauge:

I always get the same measurements on the handheld pressure gauge as I do at the filling station: 46 psi.
Now the interesting part. As we get rolling, the TPMS sensors on the Model X' Instrument Cluster ALWAYS display 42 psi, despite getting 46 psi on both the filling station and the handheld pressure gauge a couple of minutes earlier...

 

[zoomer0056]

There appears to be lots of confusion going on here regarding several different elevation/air pressure related events.

When TPMS (and other pressure metering devices) are manufactured, they are calibrated for sea-level. As these are taken up in elevation, without the ability to recalibrate, they are inherently off. At the elevation I live at (~5,000ft) this equates to almost exactly 3psi. That is to say the car reads them as 42psi even though the actual pressure of the tires is 45psi if you meter them using a device calibrated for elevation.

This means that we're in danger of trigger the low pressure warning at a very nominal 43psi. Keep in mind that this is AFTER setting proper tires pressure based on devices calibrated to this elevation. The TPMS will display 3psi lower than what the tires are actually at. This will then vary even further if temperature changes or I drive up or down in elevation so I know what others are mentioning and I'm not disagreeing... I'm saying this is different and EVERYONE at higher elevation experiences it, whether they notice or not. I just happen to have my tires PSI in the display at all times so I'm more aware of what my pressure is when driving than the average driver.

I would be surprised if people here were driving around at that pressure regularly without thinking anything of it. Imagine if you were getting warnings all the time at this pressure level.

If I go up higher in elevation, this discrepancy increases even further and can read nearly 5psi of difference in some mountain towns we frequent. This means that even though your tires are actually at 45psi, the TPMS system in the car could actually read below 40psi which triggers the warnings even if you adjust pressure while you're up there using devices properly calibrated to that elevation. This is independent of temperature changes and the difference of the pressure as you increase or decrease elevation w/o correcting pressure which are different things that occur related to pressure and elevation.

Another example is a Dewalt inflator I own. It will read 3psi off every time I power it on because the internal default pressure metering device inside is calibrated for sea-level. There is a sequence you can do each time you power it on that causes the inflator to recalibrate for the elevation which artificially compensates for this. It artificially adjusts the reading to compensate for the difference in pressure at the given elevation. The crappy part is that it doesn't store this new calibration which means that I must do this each time I power it on if I want my pressure to be accurate.

good article from TireRack https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167

 

[zoomer0056]

Quite interesting article/thread. Here's my personal experience:

My wife and I do a Costco run every week (Not my cup of tea, but my wife loves going to Costco, so happy wife happy life!), and we either take my Model S or her Model X. (95% of the time we take her Model X). So every 2 weeks or so our routine changes to accommodate a tire check-up procedure.
Once we get there, I leave her at the store's entrance and I drive a few feet towards the tire center in order to use their complementary nitrogen filling stations:

According to our Model X' Tire and Loading Information label, our 20" slipstreams should be at 42 psi cold. We live at 8,622 feet of elevation, so that's around 4 psi of difference from sea level. We live quite close to our nearest Costco, so we just travel 3 miles from home on pure local roads, so not much time/distance/speed for tires to warm up.

So now I park at the nitrogen filling station and I set the machine to 46 psi. Once I fill all 4 tires, I move our Model X to the parking lot and join my wife inside the store. We usually take around an hour to do our shopping, so this gives the tires more than enough time to reach ambient temperature, although they are filled with nitrogen and they don't actually have a long enough trip to heat up as much.
Once out, I pack all the things in the trunk and then procede to measure the tire pressures again, but now using this Michelin Digital Tire Pressure Gauge:

I always get the same measurements on the handheld pressure gauge as I do at the filling station: 46 psi.
Now the interesting part. As we get rolling, the TPMS sensors on the Model X' Instrument Cluster ALWAYS display 42 psi, despite getting 46 psi on both the filling station and the handheld pressure gauge a couple of minutes earlier...

George ... is that you?!

 

[Ostrichsak]

Quite interesting article/thread. Here's my personal experience:

My wife and I do a Costco run every week (Not my cup of tea, but my wife loves going to Costco, so happy wife happy life!), and we either take my Model S or her Model X. (95% of the time we take her Model X). So every 2 weeks or so our routine changes to accommodate a tire check-up procedure.
Once we get there, I leave her at the store's entrance and I drive a few feet towards the tire center in order to use their complementary nitrogen filling stations:

According to our Model X' Tire and Loading Information label, our 20" slipstreams should be at 42 psi cold. We live at 8,622 feet of elevation, so that's around 4 psi of difference from sea level. We live quite close to our nearest Costco, so we just travel 3 miles from home on pure local roads, so not much time/distance/speed for tires to warm up.

So now I park at the nitrogen filling station and I set the machine to 46 psi. Once I fill all 4 tires, I move our Model X to the parking lot and join my wife inside the store. We usually take around an hour to do our shopping, so this gives the tires more than enough time to reach ambient temperature, although they are filled with nitrogen and they don't actually have a long enough trip to heat up as much.
Once out, I pack all the things in the trunk and then procede to measure the tire pressures again, but now using this Michelin Digital Tire Pressure Gauge:

I always get the same measurements on the handheld pressure gauge as I do at the filling station: 46 psi.
Now the interesting part. As we get rolling, the TPMS sensors on the Model X' Instrument Cluster ALWAYS display 42 psi, despite getting 46 psi on both the filling station and the handheld pressure gauge a couple of minutes earlier...

Your experience aligns perfectly what mine (and several others on this and many other forums who live at various elevations) and matches almost exactly what I guess yours would be off at that higher elevation. If you lived at 5k like us, you would likely see a 3psi difference from reality which is what our cars consistently ready. Has over 6 different Tesla Model S's now and the only one that didn't display was our first one that had the Gen 1 version of TPMS system so it didn't ready/display actual individual tire pressures.

Side note: I would encourage you to not intentionally over inflate your tires to try to make up for what your Tesla reads. The Tesla uses TPMS which are known to be off at elevation (3psi for us, 4 psi for you) so it's the cars themselves that are reading incorrectly. You should be able to easily replicate this with any known-calibrated air pressure measuring device. The easy solution would be for Tesla to simply introduce a single line of code that uses the vehicle's elevation to offset the displayed tire pressure. 3psi would be perfect for us @ 5k feet and 4psi for you at 8k feet. This would allow our cars to display the actual tire pressure more accurately and not force us to overinflate our tires to avoid the dreaded warnings @ 39psi that, for us, is actually closer to proper pressure than it is a dangerous condition.

good article from TireRack https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167

Again, you are confusing several different ideas surrounding elevation and tire pressure. What you linked to is NOT what I'm talking about. The very first sentence of your link reads:

"Significant changes in altitude affects tire pressures when traveling from one elevation to another."

We are NOT talking about traveling from one elevation to another. What I'm talking about is staying at the same elevation and TPMS systems displaying a consistent discrepancy to reality. What you're talking about (filling at one elevation and then driving to another) is much less of a concern than other factors. I'm not saying what you're talking about is incorrect or inaccurate, it just don't apply to what I'm talking about.

 

[Art VandeIay]

I would encourage you to not intentionally over inflate your tires to try to make up for what your Tesla reads. The Tesla uses TPMS which are known to be off at elevation (3psi for us, 4 psi for you) so it's the cars themselves that are reading incorrectly.

Funny that you read my mind ...

As I was reading your post, I was thinking about the fact that every time we take our Model S or X to our local Service Center, the TPMS readings on the Instrument Cluster always display 45 psi.

I guess the Service Technicians over-inflate the tires in order to reduce the probability of customers getting a low tire pressure warning at our elevation, which means that tires must be actually at 49-50 psi of gauge pressure!!!

 

[Barry]

What's the big deal about the low pressure warning if you know it's meaningless in this context?

There are plenty of times I'm in the foothills or mountains during the cold weather season and I get to the car and it reads 38 or 39. Yeah, it's 20-30F colder than my usual haunts and the pressure will be "normal" when I get home.

 

[Ostrichsak]

What's the big deal about the low pressure warning if you know it's meaningless in this context?

There are plenty of times I'm in the foothills or mountains during the cold weather season and I get to the car and it reads 38 or 39. Yeah, it's 20-30F colder than my usual haunts and the pressure will be "normal" when I get home.

The big deal is that 1) it conditions you to simply ignore warnings that were created to bring your attention to potentially dangerous situations and 2) it's just annoying af to have an error message on your main screen as well as your instrument cluster. I'm only surprised that your suggestion isn't to just put a piece of black electrical tape over the error as a fix.

In your scenario it's different because you know you'll be returning so there's no sense to adjust and you can just ignore it, if you want. That said, 39 sitting cold in the mountains isn't too bad since as soon as you start driving and get a little heat in the tires you'll get 1 to 2 psi of actual pressure gain pretty quickly. Sounds like you'll get the rest back once you get home as well.

This is far different from what I'm talking about though. I'm talking about a consistent 3psi off at this elevation no matter what you do. Since they've chosen to set the low pressure threshold for the warning, this means that you literally have 3psi of buffer between proper pressure and a low pressure warning. That's an annoying dance to do just because Tesla refuses to compensate their reading for elevation which would be childes plan in terms of coding.

Personally, I'd still make efforts to keep your tire pressure at a consistent 45psi as much as possible for safety and proper tire wear but that's just me.