My Car Has An Altitude

[jwanderson88]

The altitude where I live is 5600 feet. It looks like the air pressure here is around 12 psi. At sea level it is about 14.7 psi. This causes problems with devices that measure tire pressure which are calibrated for sea level. My dashboard graphic says that the pressure is 44 psi in all my tires except one, which is 41 psi (low). My standard tire gauge says they are around 48 psi. The air compressor in the bed of my R1T says the tires are 52 to 55. The fact that the dashboard is low makes sense. But the truck bed compressor is way off. I used it for the first time yesterday and noticed that the reading was off. I would prefer to think that there is a logical explanation for this instead of that something isn't working right.

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[WSea]

I wouldn’t trust the compressor psi. Use a gauge to double check if you want a specific psi

 

[Marchin_MTB]

I have noticed this as well. I asked a service tech about the TPMS reading being high and he said all trucks seem to have this issue. He then remarked on how the compressor is also biased, but in the other direction.

Hoping that an OTA will allow us to calibrate these better in the future.

 

[zefram47]

Is your stand-alone gauge digital or analog? If it's analog, you might get another one to test against. If it's ever been dropped it's likely not reading correctly anymore. Another thing I've noticed is if I air down up in the mountains say at 8-10k ft and then come back down to my house around 6k ft I'll need to top up a bit to get back to where it ought to be. But even by my stand-alone gauge I need to do 50-52 psi on the built-in compressor for them to match.

On another note, I bought a Ryobi compressor recently with a digital gauge that can be calibrated. I used it in Denver and then drove down to Nebraska around 1100 ft and when I turned it on it showed about 2 psi until I manually recalibrated it for zero. So there may be something to Rivian not calibrating zero on the built-in compressor at power-on.

 

[Mathme]

altitude and ambient temperature will throw off the tire sensors. With fall setting in and it being cooler, it's not uncommon to need a little extra air. Even out here in California where it's pretty mild, I've had to add a few pounds of air to my tires this past week.

 

[zefram47]

altitude and ambient temperature will throw off the tire sensors. With fall setting in and it being cooler, it's not uncommon to need a little extra air. Even out here in California where it's pretty mild, I've had to add a few pounds of air to my tires this past week.

For clarity, temperature doesn't affect the sensors...it actually decreases the air pressure in the tires.

The ideal gas law is pV = nRT or p = nRT / V. With volume a near constant in the tire, the pressure is effectively changing just by temperature changes. So yes, when it gets cold outside you'll see a couple psi drop and vice versa if you last checked pressure in the cold then you'll be a few psi high when it gets back up to 70-90F, etc. One of the reasons folks do nitrogen fills is that pure nitrogen is much more temperature stable than atmospheric air, which is still kind of funny since normal air is already 78% nitrogen anyway. The water vapor and some of the other smaller constituents of the atmosphere are what affect things the most. The above also assumes that the ambient air pressure remains more or less constant...where changing elevation/altitude obviously affects that.

https://en.wikipedia.org/wiki/Ideal_gas_law

 

[dgennetten]

Is your stand-alone gauge digital or analog? If it's analog, you might get another one to test against. [...]

While digital can certainly be more precise than analog, it is NOT more accurate. Pressure is an analog phenomenon. In order to read it as digital there has to be an analog measurement, followed by analog-to-digital conversion.

 

[zefram47]

While digital can certainly be more precise than analog, it is NOT more accurate. Pressure is an analog phenomenon. In order to read it as digital there has to be an analog measurement, followed by analog-to-digital conversion.

What I meant by that is a lot of digital gauges will zero themselves when you turn them on. An analog gauge can be damaged through impact or other issues and without occasionally testing it against something else you'll never know if it's accurate. Accuracy and precision are also two different things. And sure, a digital gauge can also be damaged.

 

[Jake Stone]

I wouldn’t trust the compressor psi. Use a gauge to double check if you want a specific psi

I live in LA and recently aired up in preparation for a road trip and set the compressor for 48 psi. I got 53 psi on my gauge. the compressor was off 5 psi. Weird.

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