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Discussion Starter · #1 ·
I thought this was a simple question until I started reading. Now I'm confused.

As the ambient temperature changes (summer/winter) do I keep the tyres at the recommended pressure, or adjust the pressure so that it would be at the recommended pressure at the standard temperature.? (Assume the pressure measurement is made on tyres not driven for several hours.)

Thanks,
Greg
 

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Which tires and sizes? Street or track?

For street I aim for within 2psi of recommended cold tire pressures when tires are indeed cold and I don't pay too much attention to hot psi. Hot track pressures I look for 35psi all around after each session and don't pay too much attention to cold pressures, starting typically around 26-27 psi. RE-71r on 18" wheels.
 

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Discussion Starter · #4 ·
I have 18"--29 cold.
As in post 1--"cold" is defined as 68F/20C and not run for 2-4 hours. What do I put in when the ambient temp is 5C or 45C and the car has not been run in hours?

Greg
 

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As the ambient temperature changes (summer/winter) do I keep the tyres at the recommended pressure, or adjust the pressure so that it would be at the recommended pressure at the standard temperature.? (Assume the pressure measurement is made on tyres not driven for several hours.)
Bottom line, I don't think it matters. If you set them to recommended levels and the temperatures are't too extreme you are probably okay.

I check my tires at least once a month at whatever the current ambient is. I've heard/read that pressures change by 1# for every 10° fahrenheit degrees, which may or may not be correct. No matter, it is close enough.

Rightly or wrongly I set my pressures to the recommended level regardless of current temperature and hope temperature won't change too much before the next time I check. I don't believe this is a problem.

If you adjust for temperature and the temp is 30°F and the rule above applies you would have to set a normal 32# to 28# and that doesn't seem to be right.

Another poster said he tries to keep hot temp pressures around 35 on the track. I've found my tires on the street often get to 35-36° when hot, so that might be another way to look at it.
 

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I really wish there was a more definitive answer to this question. My 981 CS is kept outside of AZ where temperatures can run from the low 40F in the winter to +115 in the summer. So checking tire pressure at ambient seems a bit short sighted. I noticed the Porsche dealer set the pressure at +1 when I stopped by on my way home. i.e. tires were hot. Is it possible that the TPM system has temperature compensation?. Anyway I now run in comfort mode so I'm not too worried about overpressure I'd just like to know the facts about the whole pressure/temp thing.
 

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I really wish there was a more definitive answer to this question. My 981 CS is kept outside of AZ where temperatures can run from the low 40F in the winter to +115 in the summer. So checking tire pressure at ambient seems a bit short sighted. I noticed the Porsche dealer set the pressure at +1 when I stopped by on my way home. i.e. tires were hot. Is it possible that the TPM system has temperature compensation?. Anyway I now run in comfort mode so I'm not too worried about overpressure I'd just like to know the facts about the whole pressure/temp thing.
The TPM system doesnt worry about cold or hot ambient temps, so there is no need to overthink this and make it more complicated than it needs to be. In other words it doesnt matter if you have been driving the car for 100 miles in blazing heat or on frozen roads, or if the car has been sitting for days. Simply set the pressure in the tire until the TPM says "0" on all four corners at any point and you are good.

This is actually really nice as you dont have to worry about waiting until they are cold and the car has been parked for few hours in order to fill the tires like you would using a gauge. The zero baseline remains constant no matter what they expand to from that point, so the system will not suddenly say they are over or underfilled just because the temp changed after driving them.

Now having to adjust things as the seasons change is another matter, so yes you may need to add some air in the winter and remove as it gets hotter to maintain that "zero" but this is normal. I may be at zero all the way around in August, but by December this has dropped to -1/-2 so I simply adjust as needed to keep the neutral value as things change over time and dont worry about what changes occur after driving. This of course applies to normal street driving and not tracking the car where specific values are being sought after for specific performance adjustments.
 

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Discussion Starter · #8 ·
Hi all,
I kept searching for the answer because of our extreme temperature fluctuations and the more critical performance criteria than my old car. Here is what I've learned.

The recommended pressure on the door jam of the car is the “cold tyre” pressure and should be considered the minimum—that’s why you measure it after the car has been sitting for hours (over night is best, so you are measuring at the usual minimum daily temperature). Driving more than 2 Km on cold tyres will start to raise the temperature and pressure so you are no longer measuring the “cold tyre” pressure and will have to “guess” the compensation.

It is quite safe to raise the minimum by 3 psi or so if the car is heavily loaded and a long trip is planned or to ‘sharpen’ the steering. More than that raises the danger of punctures.

Driving on tyres below the minimum pressure is the major cause of excess wear, poor gas mileage, and catastrophic failures.

Tyre pressure will rise about 1 psi or 7 Kilopascals for every 5°C rise in tyre temperature, no matter what the cause. Ambient temperature, sun exposure, and driving are the main influences.

The average pressure rise from street/highway driving is about 5 psi (35 Kilopascals) over half an hour+ and this is normal and expected. As long as the tyre pressures remain well below the maximum (printed on the tyre sidewall) it’s all good.

Higher pressure does give a harsher ride and a deterioration in handling and stopping so if the rise in ambient temperature and sustained driving raises the pressure to uncomfortable levels you MAY let enough air out to reduce the pressure to a nominal 30% more than the minimum. Just remember that after a substantial stop you may need to increase the pressure as it will have dropped below the minimum.

It is best to have and use your own pressure gauge for accuracy and consistency. The ones at petrol stations can be unreliable.

Cars being used on the track will have the pressures adjusted each run.

Filling passenger car tyres with nitrogen is primarily good for the seller. It has virtually no effect at normal speeds and conditions. Even at race speeds it only makes a difference as the tyre temperatures approach 100°C as there is no moisture in clean nitrogen to expand rapidly as it boils. Aircraft use nitrogen for the same dryness—it stops the water in the tyre from freezing as the temperature approaches minus 40°.

Sources
Personal communication from an experienced race driver (no, not Sebastian Vettel)
The Tire Rack’s Tire Tech section
Barry’s Tire Tech website (He’s a tire engineer for a major manufacturer and has some very interesting and advanced information.)

Greg
 

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This is my understanding of this.

It is irrelevant if it is -15C in winter and +30C in summer.

If the cold tire pressure is set to the recommended value for whatever the temperature is outside, driving the car will warm the tires up only as dictated by those conditions. For example, if it is -15C outside it is doubtful if driving the car will raise the temperatures - and hence the pressure - more than a few psi. Setting the tire pressure when it is +30C to the recommended amount will also result in increased pressured when driving, but the starting point is now +30 and, once again, the pressure will increase accordingly and likely by a similar amount to the cold settings. This means, however, that one should set the tire pressures frequently enough that the cold pressure setting is appropriate for the outside temperatures. Setting the pressure in winter and then driving the car in summer at those pressures set in winter will result in over inflation.

I set the pressures in my Cayenne at each tire rotation to the same setting (ie 35 front, 38 rear). This is done about each 6 weeks. Obviously, outside conditions change during those intervals. The pressures while I am driving always go up over the cold pressure setting by about 3-4 psi. Now, if I should rotate my tires in May, and set the pressures accordingly, but drive the vehicle so little such that in July I haven't needed to rotate the tires, I might need to reset the cold temp pressures down a bit as the ambient temperature will have gone up by 10C or so and, accordingly, the cold pressure will have gone up also. The adjustment is then needed.
 

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If you inflate your tires with NITROGEN, they will fluctuate much less, this is why aircraft tires racing car tires use nitrogen, Costco uses only nitrogen for all tire mounts......... FYI
 

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If you inflate your tires with NITROGEN, they will fluctuate much less, this is why aircraft tires racing car tires use nitrogen, Costco uses only nitrogen for all tire mounts......... FYI
Using nitrogen to inflate racing tires makes sense, but not so for street use. My local BMW dealer uses nitrogen on all their new car tires, and charges $75 for that service. Dealerships always find ways to get money out of the pockets of their customers. Back in the 1970s Dealers use to make a bundle by offering undercoating to prevent rust. They rode that gravy train for a couple decades, until it was proven that the undercoating did harm. So now they have another money maker, nitrogen.
 

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I have always set my tire pressure more or less to the recommendation when they are cold, meaning I have to add air as ambient temps go down in the winter and let out air as ambient temps rise in the summer. What slightly confuses me is in my owner's manual it says the values on the Tire Pressure Plate "... are for cold tires (68°F /20°C )." I understand the "cold" part, but not the 68F part. I have chosen to ignore it.
 

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I have always set my tire pressure more or less to the recommendation when they are cold, meaning I have to add air as ambient temps go down in the winter and let out air as ambient temps rise in the summer. What slightly confuses me is in my owner's manual it says the values on the Tire Pressure Plate "... are for cold tires (68°F /20°C )." I understand the "cold" part, but not the 68F part. I have chosen to ignore it.
You have to have a reference point for your tire pressure recommendation. Porsche's is 68 degrees F.
 

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You have to have a reference point for your tire pressure recommendation. Porsche's is 68 degrees F.
OK, but then where do I set the pressure if my ambient is, say, 20F? Currently I am setting it to the same pressure no matter what the ambient. If that is not correct, it would seem Porsche (and other car manufacturers) would include a chart for recommended pressure vs. ambient temp.
 

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OK, but then where do I set the pressure if my ambient is, say, 20F? Currently I am setting it to the same pressure no matter what the ambient. If that is not correct, it would seem Porsche (and other car manufacturers) would include a chart for recommended pressure vs. ambient temp.
You are correct. That's why there is only one number on the door or manual. You need "more air" to get to 32 psi when its colder. Same pressure in the tire.

Re nitrogen - most of us use about 78% N. The reason to use pure nitrogen is to limit heat related pressure changes. Both oxygen and water vapour (most of the other 22% of air) cause greater pressure changes with temperature than does pure N. If it's free, I suppose its a deal, except when you need to add pressure. Air is pretty convenient and economical.
 
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