2025 RWD Tire Pressure question

[isamuels]

2025 RWD Tire Pressure Question:
My car has 21" Mission E wheels. The driver door label says cold pressures should be Front 38 and Rear 45. I checked the tires this morning and they are exactly 38/45. So I assume no deviation. However the dashboard screen shows:

I can't understand why the screen still shows these pressure deviations. Can anyone explain this?

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[figure1a]

Go into vehicle settings and see what your tires are setup for. The door jamb sticker settings are set for a full load—I think. Also, the tire settings could have been done with the wrong pressure which is why you see what you see. Also in the tire settings is where you can zero that out to current tire pressure (assuming that is what you want). I always set my pressure to what the PCM says (based on my selections) and not the door sticker settings.

 

[SergeyIndy]

^Yes to this. Set your tires to the right size, type summer vs. other, and load as I do partial for 2 people and no luggage. Then use car PCM, not the sticker.

I would not drive mine if I see the numbers your car is showing.

 

[Tooney]

@isamuels To complicate things further, use this Taycan wheels/tires TSB (rather than the door jam label) to figure out what correct tire pressures should be. Start on page 13, and check for type of tires, and wheels, etc. to get pressures.
This tire pressure chart should also be available in the car's PCM.

 

[freeforall]

I noticed that as well. The number that you see on the door are for a specific temperature. Cold tires doesn’t mean anything as you can have cold tires both in hot summer and cold winter. I don’t remember exactly but I believe in my car the numbers shown on the door are correct when outside temperature is between 15 and 25 degrees C.
I stopped looking to those numbers and I just adjust my tire pressure based on the deviation displayed on the dashboard. In my case the recommended pressure varies by 0.3 bars between hot summer and cold winter.

 

[f1eng]

2025 RWD Tire Pressure Question:
My car has 21" Mission E wheels. The driver door label says cold pressures should be Front 38 and Rear 45. I checked the tires this morning and they are exactly 38/45. So I assume no deviation. However the dashboard screen shows:

I can't understand why the screen still shows these pressure deviations. Can anyone explain this?

Looks like the tyres are pressured for full load and the PCM setting is light load.

I set my pressures and PCM for heavy load when I went on holiday 4 up and luggage this summer and that is about the difference in pressure when I set the PCM back to light load before I let the tyres down.

 

[Speuk]

Looks like the tyres are pressured for full load and the PCM setting is light load.

I set my pressures and PCM for heavy load when I went on holiday 4 up and luggage this summer and that is about the difference in pressure when I set the PCM back to light load before I let the tyres down.

Frank do you make an adjustment for setting tyre pressures in cold weather. Porsche TSB say pressures are at 20°c so setting at say 10°c would you lower pressures by say 0.1bar?

 

[f1eng]

Frank do you make an adjustment for setting tyre pressures in cold weather. Porsche TSB say pressures are at 20°c so setting at say 10°c would you lower pressures by say 0.1bar?

I adjust by the increment shown on the app.

 

[Speuk]

I adjust by the increment shown on the app.

Thanks Frank, that's what I've been doing on my 4S CT with 20" rims. On the new 4S ST with 21" rims that seems a big adjustment but probably correct. It will be interesting to see if that matches the spec pressures when the ambient temp is 20°c.
When i collected the car the tech who prepared it (needed a new hv charger before delivery and ibasked they checked the tracking) said keep the tyres correctly inflated. I then drove off and the App showed +0.4 bar when I got home ?

 

[f1eng]

Thanks Frank, that's what I've been doing on my 4S CT with 20" rims. On the new 4S ST with 21" rims that seems a big adjustment but probably correct. It will be interesting to see if that matches the spec pressures when the ambient temp is 20°c.
When i collected the car the tech who prepared it (needed a new hv charger before delivery and ibasked they checked the tracking) said keep the tyres correctly inflated. I then drove off and the App showed +0.4 bar when I got home ?

I find it eventually but there is a menu buried in the PCM which allows you to select wheel size, winter or summer tyres and high or normal load.
If that is set correctly I am assuming the display gives valid data.
When I got mine I set the cold pressures from the door jamb sticker and the display showed within 1 psi after that so I was reasonably confident that is how it worked.

Slightly irrelevant anecdote:-

Psi is one of the very few Imperial units I still think in. In racing tyre pressure is the single most important thing to have right, both for performance and reliability. I targeted within 0.1 psi. It was one of the things I taught race engineers and new drivers as soon as I could, amazingly to me, quite a few drivers had never known and were often sceptical. Amazing to get as far as F1 and not know this, but I am amazed still watching interviews how rife ignorance is in the less well performing F1 teams even today but it has always been the case "those that know don't say and those that say don't know" in motor sport. I would not have written this if I were still involved.

One of the race engineers I trained (in 1987) ended up Ayrton Senna's race engineer at McLaren and I was pleased to hear Ayrton would not even discuss how the car was handling until the tech had reported the tyre pressures and wouldn't adjust the car if they were wrong, getting them corrected then running again. It is often frustrating with self-confident novices.

 

[Speuk]

I find it eventually but there is a menu buried in the PCM which allows you to select wheel size, winter or summer tyres and high or normal load.
If that is set correctly I am assuming the display gives valid data.
When I got mine I set the cold pressures from the door jamb sticker and the display showed within 1 psi after that so I was reasonably confident that is how it worked.

Slightly irrelevant anecdote:-

Psi is one of the very few Imperial units I still think in. In racing tyre pressure is the single most important thing to have right, both for performance and reliability. I targeted within 0.1 psi. It was one of the things I taught race engineers and new drivers as soon as I could, amazingly to me, quite a few drivers had never known and were often sceptical. Amazing to get as far as F1 and not know this, but I am amazed still watching interviews how rife ignorance is in the less well performing F1 teams even today but it has always been the case "those that know don't say and those that say don't know" in motor sport. I would not have written this if I were still involved.

One of the race engineers I trained (in 1987) ended up Ayrton Senna's race engineer at McLaren and I was pleased to hear Ayrton would not even discuss how the car was handling until the tech had reported the tyre pressures and wouldn't adjust the car if they were wrong, getting them corrected then running again. It is often frustrating with self-confident novices.

Yep the PCM is set correctly for 21" summer tyres a d partial load. I toggled to 20" and back to 21" just to force a remeasure. The facelift models have a different TPMS valve not that should make any difference
I check my tyres and pressures at least once a week... ?

 

[RAHRCR]

If you set your cold temps to what is shows on the door jamb, you are good. You can ignore the PCM. The key is making sure it’s set when them when they are cold and to check/adjust as the weather changes.

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