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Discussion Starter #1
I also asked Toyota about this: My Highlander had a way to reset the tire pressure system so as to teach it that one was at altitude (I live close to 10000 feet). Without doing that the air pressure readings on the display will be wrong, as they assume 14 lbs ambient air pressure, when it is only 10 lbs up here. Toyota says there is no such reset on the RAV4. Too bad. What this means is that I will eventually get low pressure warnings when there is in fact 4 lbs more pressure than the sensor thinks. Well, the old-fashioned pressure gauges will tell the truth. At least this Toyota has sensors for each tire, which is an improvement on the Highlander's single pressure warning with no indication of which tire.
Tire pressure at Altitude
This is from another thread, but it seems like it might deserve it's own thread. So I tried to look it up, you know, on the internet. O.M.G. Sheer insanity out there. Clearly some folks did not pay attention in high school physics or chemistry. But anyway, it does seem like a valid question, as to how the TPM system in a car deals with altitude. As best as I can tell, you don't need change your tire pressure with altitude. It will, however, change itself. So yes, if you drive to Denver from Seattle, you tire pressure will be higher in Denver, and, (assuming you are going to stay), you should adjust your tires back to the factory numbers.

Looking in the owners manual, I don't see altitude mentioned anywhere in relation to tires. Initializing the tire pressure warning system does look long and complicated. But as far as I can tell, it does not calibrate the system. It only appears to set tire pressure reference level. That is, it sets the notice/alarm levels, based on the pressure in the tires when you started the initialization. Anyway, my guess is that the instrument panel indication of the tire pressure is always correct, and does not need to be reset for different altitudes. Nor would the tire pressure monitoring system. If it was set to 32psi at sea level, it should still be set to 32 psi in Denver.

No?
 

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The pressure your tires feel, is really just the pressure inside the tire, minus the pressure of the external environment. In higher altitudes, there's less pressure in the external environment, therefore the difference between inside and outside of the tire is greater, and if the temperatures are the same, your tire will appear to be inflated at a higher pressure the higher up you go. Not sure how the TPM measures in the R4P (is it an aboslute, or relative measurement). At the end of the day, the only tire pressure value you should really trust is the one a good quality analog/digital gauge will give you. I keep an analog one in each of our cars so I don't need to worry about a dead battery, and a nicer digital one at home for top offs.
 

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Thanks for promoting my post! Here is my understanding. The tire pressure sensor measures the internal absolute pressure inside the tire. But the gauge pressure is the number that controls how the tire relates to the road. This gauge pressure, for GM, is computed by simply subtracting 14.5 from the absolute pressure. So it is very wrong at 5000 ft or, where I live, 10000. For the Bolt the only solution to avoid warnings is to overinflate a little bit. And I am told by a tire shop that this happens a lot here in Summit County, Colo. Now, when I was getting warnings on my Highlander, I called a dealer and was told about a reset procedure that would take care of it. It was very simple, and worked. But there is not such similar reset procedure for the Rav4 Prime. So at some point in a few months I might get a warning, and then I'll go around with the traditional gauge and check the gauge pressures and adjust. By the way, I believe the plug-in inflating devices -- I have one -- are also not taking ambient pressure into account, so if it says 40 on one of those as one is inflating, really the gauge pressure (up here) would be ...44 b/c the device is subtracting 14.5 when it should subtract 10.

Speaking of altitude, when I reported the mi/kWh I was getting on my Bolt (4.3 or 4.4) on a forum, it was somewhat greater than most people were seeing. Several responders said that the lower air resistance here at 10000 ft is a huge factor and will lead to improved mileage. This will of course apply to my Rav4. I am easily getting 42 miles on a charge, and I imagine that will go up in summer.
 

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Discussion Starter #5
Well, I don't know how to prove it, but I'd be surprised if the TPM system on a modern toyota read in PSIA. First of all, any modern car knows the barometric pressure, so correcting for altitude is trivial. The manual makes no mention of altitude, or how to correct for it, which would be needed if the system measured in (uncorrected) PSIA. A system that measured in PSIA, corrected to sea level for the display is just dumb.
 

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Well, that would be great and yes, barometric pressure has been very very easy to measure for many years now. But in my conversation with GM engineers of the BOLT (on another matter) I just happened to mention this issue and not only did they agree that this would be a problem, but they said it never occurred to them to correct for this and the issue applied throughout all GM models, not just the Bolt. Well, the way to prove what you suggest is to wait until I get a low tire pressure warning and then go out and measure the gauge pressure and see if it agrees with the warning. Alternatively, I can go out tomorrow and measure the pressure in a tire and see if it matches the number on the display. Those two things very definitely did not match on the Bolt. Worth doing as it is easy. Maybe that tire pressure reset system is meant for the situation where one replaces the 33 psi tires with tires that want, say, 37. Then that complicated reset method in the manual will tell the warning system when to warn so as to be correct for the new tires.
 

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Discussion Starter #9
So the Tire Rack article seems to say that 1- tire pressure will rise as you go up in altitude. 2- the temperature will likely drop. 3- end result, it does not really matter, they will likely cancel each other out.

The Coyote article has So. Many. Words. He talks about using a gauge that reads in PSIA. He talks about what pressure is equivalent to 10 psig (sea level) when you are at 7800' He then never answers the question, but maybe he is saying use 13 psig (at 7800')? It's not really clear. And in any case, I think if you need 10psig at sea level, you also need 10psig at 7800'.
 

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The comment about temperature cancelling air pressure change (as a general rule) does make sense, but is not relevant to the question for someone who stays at 10000 feet and wonders if the displayed numbers are accurate. I will check using pressure gauge.
 

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Experiment done. The display says that all 4 tires are at 31 psi. The car is in garage at about 45 degrees. Taking a traditional gauge to one of them got a 33.5 reading. The recommended pressure is 33. If we assume that 33 was the pressure at delivery, then this is consistent (I am at 9500 ft, but it is cold). But the 31 reading indicates that there is no adjustment for altitude. The suggestion that an electronic barometer is in all cars is too optimistic, as this affects only a very small %age of owners. So what will happen is that at some point the meter will read 29 or so and I will get a warning even tho the cold pressure is 31. So I will inflate, but I will overinflated a little, as is traditional up here to avoid these warnings. It is not a big problem, but the Highlander had a very easy reset method that the RAV4 does not. And it is true that 33.5 - 31 is not the 4 that I would predict from 14 less 10 (the ambient pressure difference), it is enough to convince me that it is behaving as I say.
 
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