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Old 10-30-2017, 11:01 PM   #1
fanatichuman
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Right tires on my wheels? How much PSI?

Hi all,

I have these tires on my 2001, 330xi (see pics).

They are 215/45R17 91H (see pictures).

First, I am wondering if these are OK on my car ( I bought it quite recently and get now more into making sure all is fine).

Reason I got started with the tires is that I wanted to pump them up, but did not find out to how much pressure I should pump them up.

->The note in the door only mentions 205, 225, and higher (see picture).

So, to the question if the tires are fine in general I would like to add the question how much PSI I should put on them.

Thanks a lot!
Cheers
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Old 10-30-2017, 11:12 PM   #2
micky99
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They should be a good replacement, I believe they're interchangable with 205/50 17. There's a website called tire size calculator, check the difference. Shouldn't be more than 2%.
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Old 10-30-2017, 11:42 PM   #3
fanatichuman
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Thanks! Next closest would actually be the 225/45/r17. Not sure what the 91H means though. Would you just put the 35 and 43 PSI? Cheers
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Old 10-31-2017, 12:27 AM   #4
ewcmr2
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Door card pressuresxwill be fine. 91 is weight capacity and H is speed rating.
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Old 10-31-2017, 02:21 AM   #5
Knight
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The factory tire size for those wheels is 205/50R17. You are slightly off, but nothing drastic.

As for tire pressures, there's mixed information out there. Most shops have either 32F/38R PSI or 39F/46R PSI in their database. Personally, I run 40 PSI even.
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Old 10-31-2017, 08:15 AM   #6
wasp9166
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sticker on door , you want to use the "load up to 4 persons option "


mine is 32/f, 37/r
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Old 10-31-2017, 08:24 AM   #7
apseagle
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If you do not want to think about it, the pressures on the door are fine, you will never have an issue. If you want to play around a bit and have a compressor in your garage for quick changes, you can alter it a bit. I have an 02 325Xi with the same wheels. I am running the 225 tires (Continental DWS). I am anal about tire pressures and have been playing with them a bit over the last year. For me, my driving style, and the ride I wanted I settled on 34/36 F/R. If I go for a long highway run, I may go up a tad, 36/38.
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Old 11-02-2017, 10:24 AM   #8
GhostFlame
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Quote:
Originally Posted by Knight View Post
The factory tire size for those wheels is 205/50R17. You are slightly off, but nothing drastic.

As for tire pressures, there's mixed information out there. Most shops have either 32F/38R PSI or 39F/46R PSI in their database. Personally, I run 40 PSI even.
40 PSI seems quite a bit on high side, and I imagine 46 is recommended for a fully loaded vehicle only.

Have you notice additional inside tire wear over time?
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Old 11-02-2017, 11:45 AM   #9
apseagle
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In my former life, I did 16 years on tire pressure monitoring systems research and development for the high end RV and commercial industry. I spent more time taking tire data than I care to remember. While this was for the commercial industry, all the information is completely transferable to our cars. So take these ramblings as you choose, just know that I have "been there, done that" in this department, and am a patent signer to prove it. This is just random tire pressure related information. Use it as you wish.

The air pressure in a tire is dependent on load. Period. The more air in a tire at any given load reduces sidewall flex, and reduces contact patch of the tire. Less flex and less contact patch means less rolling resistance along with less traction with the ground and compliance over rough terrain. So there is a bit of a juggling act going on there. Less pressure, means more traction, smoother ride, but at the same time more heat and more sidewall flex (less responsive).

Do you all remember the Firestone/Ford Explorer tire fiasco in the early 90's where everyone blamed Firestone for the sidewall blow outs and resulting roll overs? Well, Firestone was only partially to blame. This was a period where SUV's were coming into popularity and the manufacturers were trying to make them ride more car like. Ford, in their infinite wisdom recommended ultra low tire pressures on their door placard to make the suv's ride nice and plush. I am talking about 26-28 psi on a 4,000+ pound SUV. This ended up generating too much sidewall flex resulting in heat degradation over time and the resulting failures.

I say all this just to illustrate that choosing tire pressures, and how the manufacturers come up with a number is not just a simple data point. They have to look at the loads of the vehicle, the intended purpose, the ride they desire to achieve, the contact patch of the tire, etc. BMW (as well as other European performance marquis) tend to choose numbers based more on performance because of the car's capability and the high speeds of the Autobahn. You will be perfectly safe running the pressures recommended on their placard.

Ok, lets talk about tire temperatures in relation to pressures. First and foremost, what is meant by cold inflation pressure? It is the pressure you put in the tire when cold, right? Yes, that is correct, but.... there is more. Cold, per industry standards, is 65 degrees F. As the temperature increases, either due to ambient temperature, sun shining on it, or driving it, the pressures increase in the tire. Roughly 1-2 psi per 10 degrees F of temperature change. So, you are checking your tires before you drive the car, but it is 96 degrees outside. Now what? Well, you need to put roughly 3-5 psi more in the tire than what the CIP is listed as. What happens if you check your tires in the dead of winter when temps rarely crack above freezing? The industry recommendation is to put them up to the CIP, because there is a minimum air pressure you never want to fall below no matter how cold it is.

I can not end this without talking about the Nitrogen (N) services places are trying to sell you. They give you all kinds of hot points about how good it is. Most of that is complete hogwash meant only to separate you from your money. The rest, while based on slivers of truth, when put into real world perspectives is not worth the paper they are written on. The air you breath now is 78% nitrogen. So, unless they pull a vacuum on your tires (they don't), they are not even installing a 100% pure N fill. They claim that N does not increase pressure or not as much as regular air. FALSE. Unless they found away around Boyle's gas law. Besides, remember that the air you breath is 78% N anyway, so getting a few extra percentage points of N is not going to make any difference. They also say that the rubber in the tire does not degrade as fast with N. Again, false. Most tires are synthetic anyway. You will wear them out much faster than any internal degradation. They say that you will not loose tire pressure as fast because of molecule size. There is a shred of truth here, but again, 78%.... The difference is noise.

The one area where they are right is this. N is 100% dry. No moisture in the gas at all. If you live in a humid area and the tire shop does not drain their air compressor tank regularly and/or have a dehumidifier on their air system, this could be an advantage. But in all practices, not enough to make me want to pay for their magic smoke.

Then tell me why professional race teams use N in their race tires, it must be good, right? Yes, but for different reasons. First and foremost for many teams is convenience. A single bottle of N is convenient. It fills a bunch of tires, it is cheap, does not require power to run a compressor, and there is no moisture in it. High end teams like formula 1 also gain another benefit. It is absolutely consistent. Because of humidity, their tires can act differently if filled from a compressor in Dubai vs. Brazil or soggy England. With N, there is no moisture, thus no change.

There you have it. Now, pick a pressure you like if you want to play, or just run what the sticker says and do not worry about it and go drive.
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Old 11-02-2017, 02:07 PM   #10
Knight
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Quote:
Originally Posted by GhostFlame View Post
40 PSI seems quite a bit on high side, and I imagine 46 is recommended for a fully loaded vehicle only.

Have you notice additional inside tire wear over time?
I do a lot of high speed highway driving and keep the tire pressure a bit higher because of that. Had a few MB manuals from prior cars that instructed to raise the tire pressure by 5-7 PSI for "extended highway driving" so the habit stuck.

My tires have worn evenly from new down to 4/32nd's. The only time I noticed uneven wear was after that in the very final stretch of usable life.

I should note that I have moved away from the factory staggered setup. Summer set is 245/40/17 square and the winter set is 205/50/17 square.
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Old 11-03-2017, 01:52 PM   #11
fanatichuman
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Thanks again to all. This ended up being a very informative thread!
Cheers
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