Educate me how Tesla Models TPMS works.

[topboxman]

I am trying to understand how Tesla TPMS works.

1) The recommended tire pressure for my early 2020 M3 SR+ and 19" wheels is 42 PSI for all 4 tires according to the sticker next to driver's side door jamb.
2) I purposely pump all 4 tires to 3 PSI higher than recommended values to 45 PSI. I used a digital tire pressure gauge from https://www.amazon.com/gp/product/B07KY8RFKC/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1 .

3a) I selected "Factory Reset" via "Service" => "TPMS Sensors" and the UI screen showed recommended tire pressure of 42 PSI for both front and rear tires just like the sticker next to door jamb.
4a) I drove the car for 15 minutes and the Tesla UI showed 41 to 42 PSI for all 4 tires but never ever reached to 45 PSI to match my digital tire pressure gauge.

3b) If I selected "Learn TPMS Sensors" via "Service" => "TPMS Sensors" and the UI screen showed some random numbers recommended tire pressure between 38 PSI and 40 PSI for front and rear tires.
4b) I drove the car for 15 minutes and the Tesla UI shows 41 to 42 PSI for all 4 tires but never ever reached to 45 PSI to match my digital tire pressure gauge.

So I'm not understanding why Tesla UI never display close to 45 PSI to match my digital tire pressure gauge after driving for 15 minutes.

The outside temperature was 31F while driving.

Bottom line, Tesla UI tire pressure reading never ever showed the same or higher tire pressure reading than the measured tire pressure with a digital tire pressure gauge.

What's the difference between "Factory Reset" and "Learn TPMS Sensors" for calibrating TPMS?

Peter

 

[derotam]

You are assuming that your pressure gauge you got on Amazon is actually as accurate as it says it is. Same with the TPMS sensors.

Do you have another pressure gauge somewhere? You should compare multiple pressure gauges to make sure they mostly match each other. Then you can more easily say that the TPMS sensors are wrong.

 

[Chisale]

I agree with derotam. I've seen tire gauges that are off by as much as 10%, especially the cheap "stick type". Plus many are not particularly accurate at higher values or if they have been dropped or after many uses. Plus the TPMS sensors aren't particularly accurate either (though mine are usually within 1-2 psi). They are more designed to alert you to a low pressure situation than give an accurate result. If you're worried, then I would buy a $30-50 good dial gauge and don't get too concerned what the TPMS are telling you.

 

[heavyD]

Totally normal at freezing temperatures. Once temperatures rise you should see closer to 45 psi from the sensors.

 

[derotam]

Totally normal at freezing temperatures. Once temperatures rise you should see closer to 45 psi from the sensors.

Not the point of the OP's post.

Of course the pressure is going to rise if temperature goes up, and it will go down as temperature goes down. The OP is trying to say that his gauge is not reading the same as the TPMS sensors, which is probably just an accuracy issue.

 

[topboxman]

Totally normal at freezing temperatures. Once temperatures rise you should see closer to 45 psi from the sensors.

I'm aware that as temperature rises while driving which makes the tires warmer, the pressure rises. But I pumped all 4 tires to 45 PSI while tires were cold and the tire pressures didn't even come close to 45 PSI.

I agree with others that no two tire pressure guages are alike like no two snowflakes are alike. I'll just treat the Tesla tire pressure readings as a guide to make sure they're not abnormally low. It also seems pumping tires to high values don't work well for TPMS.

I do have another analog tire pressure gauge and it's 2 PSI lower than my digital gauge.

Which configuration would you choose? "Factory Reset" or "Learn TPMS Sensors" for calibrating TPMS?

I'm not worried about, just trying to understand.

Thanks,
Peter

 

[DeltaOne]

I agree with others that no two tire pressure guages are alike like no two snowflakes are alike. I'll just treat the Tesla tire pressure readings as a guide to make sure they're not abnormally low. It also seems pumping tires to high values don't work well for TPMS.

I use the Tesla tire pressure readings as a guide.

Check 'em when first leaving the house. Let's say they read 39. On a subsequent day, before driving, I'll take a reading with my handheld gauge and small portable compressor...and just add three pounds to whatever the reading is. That way the Tesla reads 42.

The TPMS is probably more accurate than my handheld gauge and cheap portable compressor.

 

[heavyD]

Not the point of the OP's post.

Of course the pressure is going to rise if temperature goes up, and it will go down as temperature goes down. The OP is trying to say that his gauge is not reading the same as the TPMS sensors, which is probably just an accuracy issue.

Yes and I'm saying it's totally normal. Once the temperatures rise the TPMS will read closer to what his tire gauge indicated. TPMS sensors are calibrated at 20 deg C.

 

[topboxman]

Yes and I'm saying it's totally normal. Once the temperatures rise the TPMS will read closer to what his tire gauge indicated. TPMS sensors are calibrated at 20 deg C.

Interesting. What does it mean when temperature is at 0C if TPMS is calibrated at 20C? Is there a formula about this?

Peter

 

[heavyD]

Interesting. What does it mean when temperature is at 0C if TPMS is calibrated at 20C? Is there a formula about this?

Peter

I'm sure there's formulas as BMW's for example have temperature compensation so the recommended front and rear tire pressures on the iDrive display will change all the time according to current tire pressure and ambient temperature. As far as I can see Tesla isn't doing this.

 

[derotam]

I'm sure there's formulas as BMW's for example have temperature compensation so the recommended front and rear tire pressures on the iDrive display will change all the time according to current tire pressure and ambient temperature. As far as I can see Tesla isn't doing this.

If you can find documentation for a TPMS sensor that shows a 7% error due to a 20C delta from calibration temperature reference, I would be interested in seeing that.

 

[topboxman]

I'm sure there's formulas as BMW's for example have temperature compensation so the recommended front and rear tire pressures on the iDrive display will change all the time according to current tire pressure and ambient temperature. As far as I can see Tesla isn't doing this.

I wonder that’s what Tesla's "Learn TPMS Sensors" is for? It doesn't seem to work well outside of 20C.

Peter

 

[heavyD]

I wonder that’s what Tesla's "Learn TPMS Sensors" is for? It doesn't seem to work well outside of 20C.

Peter

I have yet to switch tires on my Model 3 but on most cars Learn is used when changing wheels and pairing new TPMS sensors. If there's a discrepancy between TPMS and your tire gauge, always go with the tire gauge providing it's a good quality gauge and not the cheap pencil gauges. I have a high accuracy digital gauge below.

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We had a cold snap a month ago and my tire pressures dropped prompting the alarm so I pressured every tire to 43.0 psi according to my Longacre gauge. Needless to say after driving away in the cold the Tesla TPMS readings were 39/40 psig so 3-4 psig lower than what I just filled the tires at. Next morning when I checked the tire pressure in my heated garage the TPMS readings were bang on at 43/42 psig. If you have faith in your tire gauge then don't worry what the TPMS says especially in these cars.

 

[topboxman]

I have yet to switch tires on my Model 3 but on most cars Learn is used when changing wheels and pairing new TPMS sensors.

And rotating tires.

Peter

 

[parrainx]

Have you tried to set the pressure to 35psi and see if Tesla sees it as that? 15mins of driving highway or otherwise will not do much to increase the tire pressure. 20 minute track session in 100 degree weather, now that’s a different story.

 

[topboxman]

Have you tried to set the pressure to 35psi and see if Tesla sees it as that? 15mins of driving highway or otherwise will not do much to increase the tire pressure. 20 minute track session in 100 degree weather, now that’s a different story.

No, I've not tried it. From other responses, I'm satisfied as it is.

Peter

 

[MikeGracz]

Interesting. What does it mean when temperature is at 0C if TPMS is calibrated at 20C? Is there a formula about this?

Peter

Hmm, a nice exercise. There is a formula, the ideal gas law:

PV/Tn = PV/Tn

where P is pressure, V is volume, T is temperature, and n = molar amount of gas.
T is in Kelvin (degrees C + 273) and P is in kPa. One kPa = 0.145 psi...

Therefore, for 42 psi:
convert 42 psi to kPa by dividing by 0.145 kPa/psi: 42/0.145 = 289.655 kPa,
Assume that V and n will cancel each other out in the equation (remain constant). Plug the converted pressure value into the first half of the equation:
289.655/T.
T is in K. C + 273 = K.
At 20C, 20 + 273 = 293K
Again, assuming V and n will cancel (remain the same), the first half of the equation becomes 289.655/293, which equals 0.9886.

The new (unknown) P at 0C would be divided by 273K (0C + 273),
Therefore, the equation is now (assuming that V and n remain constant):
0.9886 = P/273
which is the same as 0.9886 times 273,
which equals 269.8833 kPa.
Multiplied by 0.145 to convert back to psi
gives 39.133 psi at 0 C (273 K) assuming only a change in temperature, and no change in volume or the amount of air (gas) in the tire....

I think that the TPMS triggers a warning if there is greater than some percentage difference among the tires, like 10 or 20%. So if all of the tires experienced the same change in temperature, they would all remain within 10 or 15% of each other and not trigger a warning...

 

[Sophias_dad]

I think that the TPMS triggers a warning if there is greater than some percentage difference among the tires, like 10 or 20%. So if all of the tires experienced the same change in temperature, they would all remain within 10 or 15% of each other and not trigger a warning...

No, you are thinking of 'differential' tpms where they don't even have sensors in the wheels, but rather differences in wheel speeds.

TPMS alerts will go off at some differential from 'expected' pressures, lets say 20%, even if all the tires reach that 20% at the same time.

 

[MikeGracz]

No, you are thinking of 'differential' tpms where they don't even have sensors in the wheels, but rather differences in wheel speeds.

TPMS alerts will go off at some differential from 'expected' pressures, lets say 20%, even if all the tires reach that 20% at the same time.

ah, thanks. Yes, that got me to read the "final rule" on TPMS from the US NHTSA, which states that the sensors must give drivers "...a warning when tire pressure is 25 percent or more below the vehicle manufacturer’s recommended cold tire inflation pressure (placard pressure) for one to four tires." So for 42 psi: multiplied by .25 = 10.5, 42-10.5 = 31.5 psi...

I have been getting warnings when all tires are reported (and measured by my pressure gauge) as being between 40-44psi, so something's off...