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4K views 19 replies 7 participants last post by  jim@tirerack 
#1 ·
Tires, forget about the car model, would always say 32/32 cold (or whatever the pressures should be). The Porsche manuals say, for example, 32/36 cold at 68F. I understand this doesn't mean hot pressures after the tires heat up, which can be 5 psi higher.

Some of these tires are so sensitive that in the winter, if one side of the car faces the sun and the other the shade at 45 degrees, one side has higher pressure. I know the TPMS "compensates" for the ambient temperature, or I believe it does when it tells you plus or minus pressures.

So, if the car is in your driveway at 45 degrees or an unheated garage at 45 degrees and you drive off, the pressures are going to be low. As you drive, the tires heat up but the ambient temp is still 45 degrees not 68.

So is the 32/36 REALLY at 68 or REALLY at the ambient temperature?
 
#2 ·
Certainly not an expert, but I've always gone by the rule that they need to be checked often, and adjusted as necessary.

Here's a good article from Tire Rack that explains some of the stuff you're asking about.
 
#8 ·
Here's a good article from Tire Rack that explains some of the stuff you're asking about.
Thank you, the tire rack article is tires 101 I've read for decades and is correct, but not what I was asking. Thank you any way:)

As for the TPMS compensating for ambient temperature, I would like to see a citation for that, it was my understanding that the system measured the pressure in each tire and reported it back, raw data, no adjustment.
Sorry, I mistyped ambient when meaning hot (hot on my brain). Page 106 | 2012-2016 911 Manual | Porsche iManuals

The displayed pressures take into account the tire temperature.

In other words, in the +- screen, it knows if the tire is hot or cold.

It's just a reading of the pressure, but there is "compensation" when it comes to changes in temp from driving as far as what the car calls proper filling, but to be clear we are talking about two different data screens.
This is correct and what I was talking about. I should have said its in the +- screen, AKA Fill Info in the Tire pressure menu.

Chows - I say tires should be set at the recommended cold settings for the current ambient temperature. ... During the winter, keeping the garage at 68 F would be expensive, and if I set tires "cold" at that temperature, at -20 or -30 C they would be dangerously under inflated.
Thank You. This is EXACTLY my question. And so the answer is "Ambient" temperature not 68 degrees. This is how I've always done it. The temperature variants this month have been extreme for us. 72 Degrees one day, in the teens the next. The pressures jump all over the place. I hate Global Warming. ;) I should have known to trust what I know rather than read the manual.
 
#3 ·
After reading your post 3 times... ;)

The specification for tire pressure is when the ambient temperature is 68F. The tire pressure will vary in circumstances where there is sun on it, or it has been driven, these are variables out side of the specification.

As for the TPMS compensating for ambient temperature, I would like to see a citation for that, it was my understanding that the system measured the pressure in each tire and reported it back, raw data, no adjustment.
 
#4 · (Edited)
After reading your post 3 times... ;)

The specification for tire pressure is when the ambient temperature is 68F. The tire pressure will vary in circumstances where there is sun on it, or it has been driven, these are variables out side of the specification.

As for the TPMS compensating for ambient temperature, I would like to see a citation for that, it was my understanding that the system measured the pressure in each tire and reported it back, raw data, no adjustment.
It's just a reading of the pressure, but there is "compensation" when it comes to changes in temp from driving as far as what the car calls proper filling, but to be clear we are talking about two different data screens. One is just the actual pressure in the tire which changes with driving or cold. The other screen is the the + and - values that tells us whether the tire is either over or under the ideal values, so if that changes then the pressure is not incorrect. This is the beauty of the system as it ignores normal driving temp changes on this screen, so you set the proper filling under any situation and not just when cold.

You can simply adjust the pressures until the gauge reads "0" whether cold or just driven and that is the proper pressure the system is looking for. It won't read + 5 lbs high for example just because you drove the car and get them hot and increase the pressure by that amount. The absolute values on one screen will read the increased pressure from driving, however the values will not show as a "+" on the fill screen because the tire pressure is still correct.

Of course switching between the comfort and standard pressure modes will shift the values the car expects to see in order to achieve this "0" value.
 
#7 ·
Chows - I say tires should be set at the recommended cold settings for the current ambient temperature. When it's -10 Celsius outside here and my garage is +6, the floor is about -2 and the tires are about +3. (My laser thermometer is one of my favourite "toys.") At the above temperatures, I set my 911 tires to 34/39 and they drop about a PSI if I'm driving on snow/ice but gain a PSI or two if I'm driving on warmer dry pavement. Any time there's a 10 degree Celsius change, I recheck and adjust my tires to the new ambient temperature (easy peasy with a compressor with a long hose and a good tire chuck in the garage). During the winter, keeping the garage at 68 F would be expensive, and if I set tires "cold" at that temperature, at -20 or -30 C they would be dangerously under inflated.
 
#19 ·
My TMPS sensors (987.2S) are usually fairly accurate and consistent, usually reading 1-2psi higher than my gauge. I've seen 3-4psi higher on a side sitting in the sun and as much as 10psi pressure gain just from driving 17 miles home. It's normal.

The best time to set your pressures is in your garage, out of the sun, and after your car's been sitting overnight.
 
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