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      06-11-2019, 12:13 PM   #1
CeEl
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Tire pressure system & recommendation?

Anybody understand the TPMS display in their M40i? And can explain it to me?

Yesterday early evening, after standing on a parking lot the whole day (so tire temps where in the 90’s on one side, around 80 on the other), the display tells me front recommended pressure is 36 psi, rear 35 psi.

This morning, fresh out of the garage, tire temps about 70, it tells me both front and rear recommended pressure 34 psi??? They actually happen to be all at 34 psi (according to car TPMS).

Tire recognition is set to automatic.
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      06-11-2019, 01:09 PM   #2
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Yes, I have noticed this a well. I think the recommended tyre pressures are adjusted relative to the ambient temperature. The hotter the tyres are the higher the recommended pressure.

34 psi seems to be the normal recommended pressure front and rear at base ambient temperature. I am guessing that will be about 70F. I am also guessing that at 80F it will be 35psi and at 90F 36psi and so forth.

Just my thoughts. Not sure though.

Correction:
32psi is the recommended tyre pressure according to the door sticker. I set mine 2psi higher, ie 34psi

Last edited by Jim S; 06-12-2019 at 11:37 AM..
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      06-11-2019, 02:16 PM   #3
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Wait, are you guys saying that the TPMS accounts for the current temp of the tire when determining whether or not to flag the tire as low?
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      06-11-2019, 02:33 PM   #4
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Quote:
Originally Posted by SgtGigglebox View Post
Wait, are you guys saying that the TPMS accounts for the current temp of the tire when determining whether or not to flag the tire as low?
Yes, the TPMS valve measures both pressure and temperature and displays both on the screen.
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      06-11-2019, 03:27 PM   #5
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Quote:
Originally Posted by Jim S View Post
Yes, the TPMS valve measures both pressure and temperature and displays both on the screen.
Thats awesome! Ive never known any car to do that. No more applying kentucky windage to what a workable pressure seems right
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      06-11-2019, 03:43 PM   #6
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Yes, it is very unusual and one of the new additions in iDrive v7. The target temperature is adjusted to compensate for the increase, or decrease, in tyre temperature.
You will know that in very hot temps the tyre pressure increases quite a lot.
Great new addition in the G29.
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      06-11-2019, 03:52 PM   #7
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I should also add that the target tyre pressure also changes when you drive the car and the tyres heat up.
Also, it measures each of the four tyres but only sets the target temp for the front pair and the rear pair.

Last edited by Jim S; 06-11-2019 at 04:04 PM..
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      06-11-2019, 09:42 PM   #8
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Quote:
Originally Posted by Jim S View Post
Yes, I have noticed this a well. I think the recommended tyre pressures are adjusted relative to the ambient temperature. The hotter the tyres are the higher the recommended pressure.

34 psi seems to be the normal recommended pressure front and rear at base ambient temperature. I am guessing that will be about 70F. I am also guessing that at 80F it will be 35psi and at 90F 36psi and so forth.

Just my thoughts. Not sure though.
Thanks... so the door sticker says 32 psi. That would be at about 50F temp. Is it supposed to be 30 psi at 30F? Surely 32 if the minimum psi?

The manual does a very poor job explaining this. And I am a bit skeptical. I never heard about increasing tire pressure to account for higher temps. Except for tracking.
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      06-12-2019, 02:33 AM   #9
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Quote:
Originally Posted by CeEl View Post
Thanks... so the door sticker says 32 psi. That would be at about 50F temp. Is it supposed to be 30 psi at 30F? Surely 32 if the minimum psi?

The manual does a very poor job explaining this. And I am a bit skeptical. I never heard about increasing tire pressure to account for higher temps. Except for tracking.
The manual (and door sticker) gives the recommended tyre pressures when "cold". I suppose it all depends on what temperature is considered to be "cold". The car is built in the EU where "cold" is likely to be about 50-55F.
I would set my tyre pressures either inside your garage on a cool morning or in the shade if this is not possible. I wouldn't read too much into what the pressures should be at various temperatures. Just set the pressures at the recommended settings and let the car do the rest of the calculations.

My opinion, for what it is worth.

Hope you are enjoying your car. My car is great, just a pity about the problem with the brakes. Hoping this will be resolved and I can go on my merry way without having to worry about getting stuck in a remote place.
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      06-12-2019, 05:51 AM   #10
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Ok, thanks, I think I get it now. The recommended adjust for temps so that the tire will be at recommended when cold. If I had to add air while hot I don’t have to guess as to how much I have to add, or have to worry about bleeding the tire during hot summer months when pressure is higher even in ‘cold’ tire.

Even though in the past I would consider ‘cold’ after standing in garage over night and adjust then to recommended.
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      06-12-2019, 06:05 AM   #11
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Yes, I think you can consider cold as you say, in garage overnight. That will be your base “cold” setting.
Happy motoring.
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      06-12-2019, 09:50 AM   #12
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This morning, the tires had ambient temp of about 72F and 34psi. System recommended 32psi for both front and rear.

Yesterday morning tires were about 70F and it recommended 34psi.

I'll take German's recommendation he gave me in another forum: just ignore it.
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      06-12-2019, 11:03 AM   #13
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Quote:
Originally Posted by CeEl View Post
This morning, the tires had ambient temp of about 72F and 34psi. System recommended 32psi for both front and rear.

Yesterday morning tires were about 70F and it recommended 34psi.

I'll take German's recommendation he gave me in another forum: just ignore it.
I don't know what the other recommendation was so I can't comment on that.

Personally, I run my tyres about 2psi over the recommended pressure, so that is 34psi. Set in my garage after overnight parking.

Today when I was driving, the tyres were showing 70F temp and 35psi pressure front and rear. The recommended was 33psi front and rear. The outside temp was about 60F.

Last edited by Jim S; 06-12-2019 at 11:39 AM..
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