[Dyno]

PV = nRT => P = nRT/V

 

[TStriker]

And that T is basically equal to ambient when the car hasn't been driven, and is just sitting outside (without direct sunlight heating the tires)

Additionally, a tire pressure gauge measures the difference between local barometric pressure and the captured air in your tire. If you drive to a higher elevation, your tire pressure doesn’t increase. The outside decreased
(tomayto/tomahto)

 

[TStriker]

It is the simplest thing ever follow the BMW door sticker the engineers have done all the math for you unless your smarter then them

Except that US market cars with only one set of pressures show the recommended (safe) inflation for the conditions shown in the upper boxes, i.e. car loaded with 785 lbs of people/stuff and electrons.

In Europe, placards and owner's manuals can have load ranges and max speeds, as opposed to our one supersize fits all.

 

[Nigelz]

The BMW TPMS also has a temperature sensor, in the past only the M cars displayed pressure and temp, but it can be coded in for non M. The displayed recommended PSI takes into account the tyre temperature, so the recommended value fluctuates based on temp.

 

[Dyno]

The BMW TPMS also has a temperature sensor

That explains a lot!

 

[Pedro]

I still don’t get why iDrive and BMW app show different recommended pressure, but same measured pressure

 

[rbenjamin]

PV = nRT => P = nRT/V

You know who explains this stuff really well? Tom Brady.

 

[Dyno]

I had to think about that for a minute...

Tom is saying "But the app was giving me a different number!"

 

[generic.user]

No, the app does not show tire info...
View attachment 27949

Interesting! My app also shows tire info for each tire (correctly identifying the front and rear tires):

* Sidenote: I hate that the app shows pressure in PSI. I have it set to show bar in iDrive, which works in the car, but is not reflected in the app. The app's settings has a Units entry, but it only allows changing distance, and volume (lol)

 

[niam]

checked with the chief mech, sticker is the cold tire temp pressure, iDrive is recommended pressure of live tire status with temp taken into account. Also varies during the drive.

Exactly. So where you can also find an indication saying a little bit higher than the indicated is ok. I cannot remember where it was mentioned…

 

[kindofblue]

Also bear in mind that when the outside temps drop as they do in the fall, pressures also drop. So checking tire pressures when cold is a prudent thing to do at least twice a year (spring and fall).

 

[J_P]

* Sidenote: I hate that the app shows pressure in PSI. I have it set to show bar in iDrive, which works in the car, but is not reflected in the app. The app's settings has a Units entry, but it only allows changing distance, and volume (lol)

I have exactly the opposite problem. App shows bar and I use PSI in iDrive. Maybe we can swap. It's pretty poor that you can't choose in the app.

I assume the defaults are because NA tends to use imperial and Europe tends to use metric.

 

[ssh]

checked with the chief mech, sticker is the cold tire temp pressure, iDrive is recommended pressure of live tire status with temp taken into account. Also varies during the drive.

Yes. This is the key: it varies while driving based on tire temp. It also changes based on maximum speed. Over 100mph is a different recommended pressure than under. These are dynamic, which is why they are different from the printed information.

 

[Dyno]

Over 100mph is a different recommended pressure than under.

Hmm... I think I need to go and confirm this.

 

[rollermonkey]

'22 eDrive40 w/ M-sport and staggered19's:
Door sticker, Front: 42 Rear: 44
iDrive8, Front: 43 Rear: 49
MyBMW app, Front 42.9 psi Rear: 49 psi
Tire Sidewall Front: 51 psi Rear 53 psi

It's stupid that there is so much discrepancy.

...and 42.9? Seriously? Who has an air pressure guage that sensitive?

 

[rollermonkey]

I should have taken pictures last night when I posted that. Last night, the car was definitely recommending 43 and 49.
This morning, it’s recommending 40 in front and 46 in the back.
😵‍💫😵‍💫😵‍💫

 

[Dyno]

I should have taken pictures last night when I posted that. Last night, the car was definitely recommending 43 and 49.
This morning, it’s recommending 40 in front and 46 in the back.
😵‍💫😵‍💫😵‍💫

Sounds about right. The tires were warmer last night due to use or ambient temperature, or even sun load, so the pressures are naturally expected to be higher. In the morning, tires are cold and the pressure is lower.

The idea is to avoid needlessly chasing the pressure by adding pressure when the tires are cold/low and deflating when they are hot/high.

I performed the same task using my organic microprocessor on a previous car that had tire pressures prominently displayed. Getting in the car after work on a sunny afternoon, I often saw that the west-facing tires had higher pressures than the east- facing tires and that after driving, the rears would gain pressure faster than the fronts (RWD car).

 

[rollermonkey]

Sounds about right. The tires were warmer last night due to use or ambient temperature, or even sun load, so the pressures are naturally expected to be higher. In the morning, tires are cold and the pressure is lower.

The idea is to avoid needlessly chasing the pressure by adding pressure when the tires are cold/low and deflating when they are hot/high.

I feel like maybe I wasn’t clear? The pressures being reported did not change. The recommended pressure changed in iDrive8. If that’s somehow more accurate/ appropriate than the door sticker, why does it never match the door sticker, which is the controlling recommendation. Like it’s never even close.

 

[Dyno]

Well, I don't know what's going on with your car, but in my case I carefully set the pressures to the "door" pressures on a temperate morning before the car was driven, as is proper. The door pressure matched the app "recommended" pressure at that moment.

Lately, I've been watching the "recommended" and "actual reported" pressures in the app. Both values go up and down in response to tire temperature, but they are not perfectly in sync. For example, right now the rear recommended pressure is 46 but the actual is 48. Tomorrow morning, both numbers will be a little lower, but both may not be exactly the same.

It's pretty close, though.

From the post above (and shown below), it looks like you could use some air in the rears,

If it were me, I'd wait until tomorrow morning and set the pressures as precisely as possible to the "recommended" value and keep an eye on things.

If you are using one of those pencil gauges, be aware that there's a lot of variability there.

A couple of psi here or there is insignificant.

 

[Hinch]

Good Day.
We are getting ready for a major road trip from Florida to Vermont. 1,500 miles one way. This thread has changed topics a bit as I’m wondering about tire pressure for the trip.

Parameters are:
Summer Heat
750 lbs of cargo - I guesstimated on the heavy side (driver, passenger, stuff, and 4 pound Chihuahua)
98.27% highway driving
Driving Speed 75-85 depending on state

2023 i40, 18” wheels
Here are my current specs:
F/L 35 psi
F/R 36 psi
R/L 40 psi
R/R 41 psi
Recommended pressures are 38 psi rear 45 psi (App and Factory recommendations are the same)
Question- What should I set my pressures at?
And here’s a slight twist- For the drive home, I’ll be around 250 pounds LIGHTER.
Thoughts?
Thank You.