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?
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.
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.
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.
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.
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.
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.
I agree, set them first thing in the morning before the car and tires heat up. I would not bother with changing as the temps go up and down. A couple PSI plus or minus is not a big deal. Maybe on the track it could come into play but for the street I would not bother.
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