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Confused about recommended tire pressure

23K views 47 replies 19 participants last post by  Don Juan  
#1 ·
My car (eDrive35 MSport) came with Hancook ventus S1evo(3) tires, size 245/45/R18 (front) and 255/45/R18 (rear). The front tires are rated 100Y, the rear tires are rated 103Y.

iDrive says the recognized tires are 255/45/R18 103Y (which are only the rear ones). The app does not show any tire size information.

My confusion is around the recommended tire pressure:
  • In iDrive, it shows recommended pressure 2.4 bar for all 4 tires
  • In bmw app, it shows 2.5 bar for all 4 tires
  • On the sticker, it seems to suggest 2.4 for all 4 tires

I intend to set it to 2.4 bar as per the sticker/iDrive, but not clear why the app would show a different recommendation. Any ideas?
Also, the dealer delivered the car to me with 2.7 bar across all tires, way above the recommendation...

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#2 ·
Hi Pedro,

My car is the e40 msport, which should be heavier. My tires are currently 245/45 R18 on all four wheels.

My recommended tire pressures are 2.55 bar in the front and 3 bar in the rear. But I shoukd be heavier than you.

This does not help you directly, except to say that maybe you should check the iDrive tire settings with the dealer to ensure the sensors are reading the correct front and rear tire sizes. Also check with the dealer's service "genius", if they have such a term in Greece ( if you are there) to ensure you need the same pressure all around, with different tire sizes and axle weights.

Happy driving!

George
 
#3 ·
And if you tap on the pressure or Tires in the app? Than you can see every tire separate, is the size correct there?

But My app also has a different recommendation than the sticker
 
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#6 ·
This is my sticker
and this the app
Was the screenshot taken shortly after a drive? I know it can show higher readings/recommendations depending on temperature, which increases when driving. It is not a big difference from sticker in any case, but still a difference...
 
#7 ·
Yes the current readings are just after a 200km drive this morning, but the recommended pressures where the same yesterday morning with 5 degrees C

But will check later today, this afternoon we should get 20 degrees C and some rain.
 
#8 ·
I had similar questions. My guess is that the app (or the car's software) has the ability to estimate tire temperature based on recent miles driven, ambient temperature, etc. Then, an adjustment is made to proper cold tire pressure to give you an estimate of what it should be at that moment.

The idea is to prevent a person from letting air out of hot tires only to find them underinflated when cold.

Perhaps a bit of German overengineering, no?
 
#11 ·
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, as it says prominently on the app, tire pressure changes with temperature.

So what would be the proper pressure immediately after a 70-mph road trip? How would you do the math without data or a model?
 
#15 ·
I have also noticed this, but I don’t see that you mentioned what the sidewall pressure is?
What do we do when the door sticker and the tire itself have different pressures listed? I don’t know how much 5 psi is in bar, but that is how much lower is showing on my tire sidewall compared to the door sticker.
And my door sticker doesn’t match the iDrive screen pressure or the app pressure.
Super fun!

Someone seriously should have had a meeting to coordinate this.
 
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#16 ·
I have also noticed this, but I don’t see that you mentioned what the sidewall pressure is?
What do we do when the door sticker and the tire itself have different pressures listed? I don’t know how much 5 psi is in bar, but that is how much lower is showing on my tire sidewall compared to the door sticker.
And my door sticker doesn’t match the iDrive screen pressure or the app pressure.
Super fun!

Someone seriously should have had a meeting to coordinate this.
Please do NOT set your tire pressure to the number on the sidewall of the tire. That is the MAXIMUM the tire can handle. Trust the BMW engineers have figured out the right balance of comfort, sportiness, and tire wear. The door sticker shows cold tire pressure. That means first thing in the morning after the car has been sitting unused for 12 hours or so. Set your tires to that figure and you will be all set. My understanding is that iDrive and the app make adjustments for calculated tire temperature. As you drive more the tires heat up and pressure goes up. A 33psi cold tire pressure might be 36psi after driving for an hour or two.

Bottom line though, to respond to your original post, the sidewall pressure is the max the tire can handle before it risks a blowout. It is only relevant to you if you are buying new tires for the car. Don't buy replacements with a lower max.
 
#17 · (Edited)
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The number on the door is great, but it is a one-size-fits-all solution. For example, I received my car in the dead of winter and set the tire pressures to the "door" value. Now that overnight temperatures are approaching 60F, I find I have to let out 3 - 4 psi to get to the "cold/door" pressure.

Next fall, I will have to add a few psi. I've been doing that for about 40 years.

As mentioned, do not inflate your tires to the sidewall pressure unless carrying the maximum load (or autocrossing :)).

PS: I once set the cold tire pressures precisely before heading on a trip to the Rockies. Once there, I checked them and found that the drop in ambient air pressure caused the tire pressures to be several psi higher. Do you think the BMW app is smart enough to consider that?
 
#20 ·
The number on the door is great, but it is a one-size-fits-all solution. For example, I received my car in the dead of winter and set the tire pressures to the "door" value. Now that overnight temperatures are approaching 60F, I find I have to let out 3 - 4 psi to get to the "cold/door" pressure.

Next fall, I will have to add a few psi. I've been doing that for about 40 years.
"Cold tire pressure" really just means the the air inside the tire is the same temp as the air outside the tire. If you set the pressure first thing in the morning in a warm garage and then drive outside in freezing weather, the tires will be overinflated. "Cold tire pressure" really just means that the tires have not warmed up the air inside the tire through rotational friction.

Your observation that you need to adjust your tire pressure as seasons change is spot on. But it doesn't mean any of those settings is incorrect. "cold" does not refer to a specific temperature, but rather the absence of additional heat from use. The iDrive seems to adjust for the heat from use, so as you drive from "cold" the recommended pressure would increase.
 
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#18 ·
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.
 
#22 ·
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 :p
(tomayto/tomahto)
 
#31 ·
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).
 
#35 ·
'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?
 
#36 ·
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.
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#37 ·
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.
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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).
 
#39 ·
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,
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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.
 
#40 · (Edited)
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.
 
#41 ·
Bear in mind that the tire pressure monitor in the wheel can vary and not always be accurate. Better to check as others have said, when the tires are cold and with a digital tire pressure gauge.
 
#43 ·
Mine is 39 front / 44 rear. 19's and MSport e40

The tire pressure info has been all over the place, and the on screen ones are pretty volatile.