PP Tire pressure info...

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RatBastard

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I have a 330i PP with 18's and am wondering a couple things:


1) When is the best time to check the tire pressure? After driving a short distance or when cold? The average temp is around 50F or so during the day and 25-30F at night.

2) What is the optimal pressure for these tires on this car? The manual says 32F and 38R.


Winter tires are going on in a week or so.. Once avg temp drops a bit more, but I was just wondering when you should check pressure.

Thanks.
 
mscoins

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Even though I have an 04 330 Ci with 18" wheels and Pilot Sports, I am sure my suggestions should apply to your questions.

I check pressures when the tires are cold and before any driving during the day. As for pressures, I run the fronts at 33 and the rears at 39, as recommended by my service tech for slightly aggressive driving. Otherwise, 32 and 38 should be fine. Any lower and the tires will have less amount of tread contact with the road providing less control and more tread wear.

My 2 cents.
 
MrElussive

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Follow what the manual says, or, as mscoins said, what your service tech said. The tire pressure ratings for 32psi front and 38psi rear are cold tire temperatures. Meaning only check the tire pressures if you haven't driven for (I believe) 5 hours or so. Best time to check is before you drive the next day. Also, make sure to check your tire pressures once a month...feel free to check more often than that (I personally check every 2 weeks), but try not to be lazy about it....once a month is a good amount of time and you should really be on top of your tires' pressures. And on a personal note, I think it's a good idea to run 1psi more than what the manufacturer's specs are just to be on the safe side. [;)]
 
Average Jae

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could be due to the bigger rubbers, and because they do most of the work? (just my imagination [:p]

btw, service tech told me that i should run 32 front, and 36 on the rear. and they are "cold readings" in the morning, or before the drive.

i try to check it once every 2 weeks. (the digital tire gauge i got is one of the best investiments.) or when i feel the car steering itself. usually due to uneven loss of pressure.

every 10 degree change in temp will affect 1 psi, and tires will naturally lose a psi per month.
 
MrElussive

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The rear pressure is usually higher than the front pressure probably because most BMW's are rear wheel drive, and accelerating puts massive stress on the rear tires (due to the rear wheels doing the driving and the weight is transferred to the rear under acceleration).
 
bmwrocks

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How much does tire pressure vary from a cold tire to a hot tire? I would imagine the hot pressure would be higher.

I guess my question is this: If I check my tires cold and the pressure is say 3 psi low in my right front, then I drive to the gas station to use their pump, do I put 3 psi in over what I measure at the gas station, even if the pressure now registers higher on my now hot tire? Is it a one for one correlation?
 
mscoins

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bmwrocks said:
I guess my question is this: If I check my tires cold and the pressure is say 3 psi low in my right front, then I drive to the gas station to use their pump, do I put 3 psi in over what I measure at the gas station, even if the pressure now registers higher on my now hot tire? Is it a one for one correlation?
Click to expand...
Yes. I check cold tire pressure before my first drive of the day. When adding air, I check one tire and note the difference. Then I add that difference to all tires. The next day, I recheck and adjust to recommended pressures if necessary.
 
MrElussive

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bmwrocks, warm tire pressure is 4psi higher than cold tire pressure. You should probably just get an electric pump for your house. But if you have to use the gas station, drive around for at least 15 minutes, and then go to the gas station and add 4psi to whatever readings you get. Meaning, let's say if you need 32psi in your front tires, then when you are filling up at the gas station and your tires are warm, you should have 36psi in the tires.
 
bmwrocks

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I feel stupid but this probably means I have been driving with low pressure for a long time (like all my life). I always knew pressures were for cold tires, but I never really paid any attention to it.

OK I am investing in the pump for home, maybe. Thing is it seems like a waste of money since there is a Hess a mile from where I live with free air.

Can I assume cold tires driving one mile? Speed is 15-25 for first 1/2 mile, then 45 next half mile.
 
R

RatBastard

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bmwrocks said:
I feel stupid but this probably means I have been driving with low pressure for a long time (like all my life). I always knew pressures were for cold tires, but I never really paid any attention to it.
Click to expand...
No reason to feel stupid.. But if you must, then I am right next to you since I started this thread to get an answer to this very question. :)

Thanks to all for the info thus far.
 
bmwrocks

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Final note:
I checked my tire pressures yesterday after the car was in the parking lot for 8 hours. Fronts had 30psi (I put 32psi hot in), rears had 35psi (I put 38psi hot in). Drove it for 20 mins to the gas station with free air pump a mile from my house. Fronts had 32psi and rears had 38psi.

So I put in 34psi in front and 41psi rears. Checked this morning and they were right on the money (32 front, 38 rear)

Moral of the story is here in Central Florida on an 80 degree day ContiSport Contact 225/45ZR 17s are 2psi higher hot than cold, and 245/40ZR 17s are 3psi higher hot than cold.

O happy day!!!!
 
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SanDiegoShaun

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Just curious as to what type of tires everyone prefers. Also I have worn out the rear and replaced them with a pair of Yokohama's and left the Continentals on the front. Anyone have any experience with different brands on the front and back. I have a 2002 330ci sport
 
Average Jae

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i heard mixing brands may not be a good idea, as they have different traction/ handling characteristics and can throw your car's balance off.

check out tirerack.com's ratings. i assume it to be a pretty good ratings site. for my next set, i want to get either the miche pilots or goodyear something or another that was rated really high on that site.

as for checking tire pressure. if the car sat out in the sun for a while, the "cold tire pressure" may vary from teh side w/ the sun exposure and the side that didn't. (in my case there was a 1psi difference! how irksome.)
 
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DrWho

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Generally, the lower the tire profile, the higher the pressure. If you're running different sized tires front/back it means that the rears are winder and have a lower profile, so they need more pressure. I'm running 18 inch wheels with Falken Azenis ST-115 rubber. My car manual says to use about 35 psi front and 42 rear. The 42 seems kind of high to me, so I'm using 39 on the rears. This feels about right for the tires, with the ride/handling compromise just about spot on.
 
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