TPMS pressures don't match pressure measured with gage

[racerjim]

My TPMS readings for all four tires read either 45 or 46 psi. I checked them today with a calibrated gage and three are at 48psi and one at49 psi. The door card states 48psi, so they are inflated correctly. How do i teach the TPMS that the current pressure reading is 48 or 49, rather than 45 or 46?

Thanks,.

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[ColdCase]

Dunno, but how far above sea level are you?

The gauge is indicating tire pressure relative to the atmosphere where the TPMS sensor does not have an atmospheric reference so it indicates internal tire pressure. The difference will depend on atmospheric pressure which varies with weather and altitude. You can set it one day, but the next day it s going to be different.

At or near sea level the indications should be close.

 

[JohnnyRoaster]

I’m not sure I would trust the sensors.

 

[MassDriver]

I’m not sure I would trust the sensors.

The view with the 0s and -5 is showing the change in pressure since the last observation (so 3 tires haven't changed but the passenger rear has dropped 5 PSI). It's not some kind of "broken sensor that's showing 5 PSI of vacuum".

 

[racerjim]

Revisiting this. Got a TPMS warning (legitimate, tires were low), adjusted pressure to 49psi (using three different gages, all reading the same), the car measured the pressure at 47psi. My elevation (~1100ft above sea level) would only account for about 0.5psi of the difference. Is there a way to adjust the tire pressure measurements to align with my readings?

 

[craz8]

I would love to know how to reset these values. My 20", All Seasons should be at 41 psi for Partial Load - and indeed they currently are. However, 2 of them are reported as -1psi and the other 2 are reporting -3psi. (Note that the manual does not have a single tire configuration of 42psi, so these should be all reporting zero as the difference)

One of my wheels likes to sometimes show 39 instead of 41 (but then it only reports -1psi instead of the current -3psi it is currently displaying)

These difference values seem to be as magically generated as the Range display. But my actual PSI values do tend to be within 1 of my measurement tools at sea level.

 

[Flying4Fun]

I would love to know how to reset these values. My 20", All Seasons should be at 41 psi for Partial Load - and indeed they currently are. However, 2 of them are reported as -1psi and the other 2 are reporting -3psi. (Note that the manual does not have a single tire configuration of 42psi, so these should be all reporting zero as the difference)

One of my wheels likes to sometimes show 39 instead of 41 (but then it only reports -1psi instead of the current -3psi it is currently displaying)

These difference values seem to be as magically generated as the Range display. But my actual PSI values do tend to be within 1 of my measurement tools at sea level.

The target PSI adjustment takes into account the current recorded temp inside the tire. This might be whats comming out as not logical if you were not aware that the inside tire temp drives the PSI adjustment number.

 

[Flying4Fun]

I would love to know how to reset these values. My 20", All Seasons should be at 41 psi for Partial Load - and indeed they currently are. However, 2 of them are reported as -1psi and the other 2 are reporting -3psi. (Note that the manual does not have a single tire configuration of 42psi, so these should be all reporting zero as the difference)

One of my wheels likes to sometimes show 39 instead of 41 (but then it only reports -1psi instead of the current -3psi it is currently displaying)

These difference values seem to be as magically generated as the Range display. But my actual PSI values do tend to be within 1 of my measurement tools at sea level.

Its also not clear from what you wrote, that you were aware of the full load vs partial load setting for the tires also? if not, maybe look into this setting when you select/change the installed tires.

 

[jason4s]

The target PSI adjustment takes into account the current recorded temp inside the tire. This might be whats comming out as not logical if you were not aware that the inside tire temp drives the PSI adjustment number.

Yup this is correct. I asked Porsche directly if I should be following the doorcard pressures or the TPMS pressures and they suggested the latter as it's the most accurate.

Both the target pressure and current pressure shown by the TPMS account for temperature.

Sometimes you'll get weird readings like all tires showing the same PSI but one of them showing a deviation versus target pressure. This is because the target pressure reading for that particular tire has a different target based on the temps, usually driving around for a little bit calibrates it properly and the deviation goes away.

 

[craz8]

Here's my display

Before parking, my driving was sedate enough to assume all tires are at essentially the same temperature - and they are at the same pressure too!

So, does this indicate I need to add pressure to (a) get back to 41 (Partial Load, 20" AS), or (b) to get to 42 or 44? This discussion suggests the former (a), and 42 and 44 are not listed pressures for my tires, so (b) is not a valid config.

The docs suggest the top number is 'Current Pressure', but (a) disagrees with that, hence my confusion here.

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