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Discussion Starter · #1 ·
When I bought my 2018 Bolt last year, the TPMS readings were low by 3-4 pounds. When I had the tires rotated at 7500 miles, I asked the dealer if they could make the readings match the actual pressures. When I got the car back, they were pretty close, but still low by about two pounds. Is it possible to calibrate the TPMS system so that the reading off the sensors matches the reading from a quality tire pressure gauge? I have the relearn tool, but AFAIK that only tells the system which tire position the sensor is on.
 

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When I bought my 2018 Bolt last year, the TPMS readings were low by 3-4 pounds. When I had the tires rotated at 7500 miles, I asked the dealer if they could make the readings match the actual pressures. When I got the car back, they were pretty close, but still low by about two pounds. Is it possible to calibrate the TPMS system so that the reading off the sensors matches the reading from a quality tire pressure gauge? I have the relearn tool, but AFAIK that only tells the system which tire position the sensor is on.
I notice that your are in Santa Fe, New Mexico, elevation 7,199 feet, and base atmospheric pressure of 11.3 psi absolute.

The absolute is important because TPMS sensors are calibrated to absolute pressure. The average tire gauge measures gauge pressure, the difference between the input and the environment. (At sea level a tire with 35 psi gauge has 49.7 psi absolute [35+14.7])

In Santa Fe, the gauge will read ~3 pounds higher than absolute (14.7-11.3). The atmospheric pressure is less. The gauge will have less external pressure to refer to and will read higher.

In terms of reprogramming, the TPMS chipsets may have a calibration programmability. Infineon chips claim this [see this]. In reality, I do not think any garage or dealer can calibrate or even wants to. If the TPMS sensor meets the 5% pressure accuracy specification it's OK. [Some vehicles, such as Honda, have a drive cycle calibration that may compensate for local atmospheric pressure and tire diameter; I do not think GM does.]

Practically, tire profile and driveability depend on gauge pressure. Gauge pressure depends on temperature and altitude. Either understand there is an offset between TPMS and your locale or use a very good tire pressure gauge.

Pressure vs. Altitude: https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
 

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Discussion Starter · #6 ·
I notice that your are in Santa Fe, New Mexico, elevation 7,199 feet, and base atmospheric pressure of 11.3 psi absolute.

The absolute is important because TPMS sensors are calibrated to absolute pressure. The average tire gauge measures gauge pressure, the difference between the input and the environment. (At sea level a tire with 35 psi gauge has 49.7 psi absolute [35+14.7])

In Santa Fe, the gauge will read ~3 pounds higher than absolute (14.7-11.3). The atmospheric pressure is less. The gauge will have less external pressure to refer to and will read higher.

In terms of reprogramming, the TPMS chipsets may have a calibration programmability. Infineon chips claim this [see this]. In reality, I do not think any garage or dealer can calibrate or even wants to. If the TPMS sensor meets the 5% pressure accuracy specification it's OK. [Some vehicles, such as Honda, have a drive cycle calibration that may compensate for local atmospheric pressure and tire diameter; I do not think GM does.]

Practically, tire profile and driveability depend on gauge pressure. Gauge pressure depends on temperature and altitude. Either understand there is an offset between TPMS and your locale or use a very good tire pressure gauge.

Pressure vs. Altitude: https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
I think I understand your post. Restated, you seem to be saying that TPMS sensors are calibrated to read 14.7 psi below absolute pressure, to approximate the gauge readings that would be expected at sea level. That calibration is off by about 3 psi at 7000' altitude (I'm a pilot and I get the concept of pressure altitude!). However, they should be reading 3 psi higher than at sea level, but in fact they read about 2 psi lower. Fact is, as long as I know the relative relationship between the TPMS sensor reading and a calibrated gauge reading, I should be fat, dumb, and happy (well, two out of three isn't bad). Thanks for your explanation.
 

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Fact is, as long as I know the relative relationship between the TPMS sensor reading and a calibrated gauge reading, I should be fat, dumb, and happy (well, two out of three isn't bad). Thanks for your explanation.
You are absolutely right (no pun intended). TPMS sensors are stable, repeatable, but not necessarily accurate. If you are confident the tires were equally inflated with a good pressure gauge (gauge pressure) and the TPMS reported pressures begin to move around--check.

For some historical insight you can read the final Federal TREAD act docket that required TPMS sensors.

There were multiple opinions (See Section C: Response to Public Comments) from manufacturers, TPMS vendors, and Tire Rack. The NHTSA responded by testing over 11,000 vehicles. Results showed that about 26% of vehicles had underinflated tires, but, a P-rated tire inflated to 20psi could be run at 100% load for 75 minutes without any failure.

The result was TPMS sensors only needed to flag a 25% underinflation within 10(?) minutes of detecting the underinflation.

For technical insight, look at the specifications for the Infineon TPMS chip (p. 149, section 4.4.1). https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d462625a528f01629624b01d6346

These chips are specified (at 95% confidence limits) to have 1, 3, of 5psi error depending on temperature and absolute pressure (7kPa~1psi).

Clearly, not FAA TSO certified.
 

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I just wanted to chime in here...

How do you know your reading is correct?

I had the pleasure of having a gauge, expensive one, that had a large scale 0-50 psi, so reading was very easy. It supposed to be great.
One day I found, by accident, that my tires were overinflated. For months, almost two years. I think I discovered it on my bicycle tires when I realised the tire was very stiff at 35 psi.
I took it to my lab to get its reading against laboratory grade gas gauge (rated for Nitrogen, Helium, Argon).
I could not believe, so I tested another one. Same thing.

It turned out by using the gauge I was adding 7 psi. And the most funny thing - it was shifted by 7 psi across the whole scale.
Meaning, true pressure of 30 psi was read as 23 psi. Therefore, my bike had actually 42 psi .
In cars, I kept 32-35 psi, what was very close to max pressure of the tire.

Since then, each gauge I get I check its calibration against a good laboratory pressure gauge across several points: 0, 10, 20, 30, 40 and 50 psi.
The most recent ones I got from Sears (about 4 years ago) are electronic and they are spot on.


Having said that - your readings might be off by several reasons.
TPMS is one thing.
Your gauge - mechanical vs electronic. Mechanical should be better overall, as it might not be dependent on the ambient pressure. However, electronic, depending on the gauge - may "reset" to ambient and correct for reading.

It may work just like the altimeter using aneroid barometer in an airplane. Except the barometer converts pressure to height. When you know the ambient pressure in the area you can adjust the gauge in the panel to give you more exact height.
Same should be done with the TPMS and your gauge that you use for measuring pressure.
 
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