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Discussion Starter · #1 ·
I thought this was a simple question until I started reading. Now I'm confused. I've asked this in "another place" and got nowhere.

The temperature here can easily range from 5° to 40+°C (winter/summer) and being a desert in the summer the temperature can change from 15° to 45° in any 24 hours. That's about a 6 psi tire pressure change!

So--the question:
As the ambient temperature changes do I keep the tires at the recommended pressure (29 psi for my tires) whatever the temperature, or adjust the pressure so that it would be at the recommended pressure at the standard temperature (20°C, 68°F)?

(Assume the car is for street use and the pressure measurement is made on tires not driven for several hours, i.e. "cold" in tire parlance.)

Thanks,
Greg
 

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As long as you check and adjust the "cold" tire pressure to recommended psi regularly (let's say once a month in the morning) this should be a none issue. That way you'll adjust it automatically with the changing seasonal ambient temperatures. I was playing with the pressure on my 19"s and found that I like them best cold filled 1-2 psi over the recommended pressure on the door sticker.
 

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As long as you check and adjust the "cold" tire pressure to recommended psi regularly (let's say once a month in the morning) this should be a none issue. That way you'll adjust it automatically with the changing seasonal ambient temperatures. I was playing with the pressure on my 19"s and found that I like them best cold filled 1-2 psi over the recommended pressure on the door sticker.
This is good advice. Always do tire-pressure checks when the car and the tires are cold, if for no other reason than to establish a consistent baseline. And play with the recommended pressures; use those as a baseline as well for adjustment above or below the recommended pressures on the door sill.

... and I'll add these points:
- Keep in mind that if you run tire pressures lower, your tires will heat up quicker and (potentially) run warmer. The effect is probably negligible in a street tire, but it can make a difference depending on your habits.
- Sudden fluctuations in ambient temps can adversely affect the TPMS system, causing it to go off. In addition to manually checking pressures once a month, use TPMS to monitor them regularly and reset the TPMS at least every season (three months).
- Keeping an eye on TPMS regularly is especially important with tire sidewall profiles 40 and below -- which is anything on a 19" rim and above on our cars. Since all OEM Porsche wheels except the 20" 911 Turbo style are not forged, the risk of bending a rim and/or splitting a tire exists -- and that risk goes up with lower tire pressures on non-RFT tires.
 

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My Cayman has the 20" wheels with 35-profile tires, lower profile than I've had before. If the car isn't driven for a couple of days, they develop flat spots which take some warming up/driving to work out. So, considering methodology, I've been thinking that checking the wheel balance needs to be done warm whereas the pressure should be checked cold...

Doug
 

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I live in the desert also with similar temperatures. I prefer to increase the recommended 29 psi on my 235/45 and 265/45 18s by 1 and 3 psi, respectively. I check the TPMS temperatures daily and check with a tire gauge once a month. No adjustments are made because of ambient temperatures. However, I agree that lowering the psi on tires with a 40 or less aspect ratio is risky to the wheels.
 

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Discussion Starter · #6 ·
I notice that the reduced "comfort pressure" for the 20" wheels is higher than the standard pressure for the 18".

Greg
 

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As long as you check and adjust the "cold" tire pressure to recommended psi regularly (let's say once a month in the morning) this should be a none issue. That way you'll adjust it automatically with the changing seasonal ambient temperatures. I was playing with the pressure on my 19"s and found that I like them best cold filled 1-2 psi over the recommended pressure on the door sticker.
I too run my 19s 1-2 psi over pressure. When the car was brand new, cold mornings would set off the TPMS with the tires at sticker pressure. Running a slight overpressure seems to have cured the problem.
 

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GregW,

I believe that lower profile tires need higher pressure (psi) to support the same weight. Think of road bike tires vs mountain bike tires. A typical one inch wide road bike tire requires two to three the psi than a wider mountain bike tire to support the same amount of weight. Any Physics majors out there who can explain this in greater detail?
 

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GregW,

I believe that lower profile tires need higher pressure (psi) to support the same weight. Think of road bike tires vs mountain bike tires. A typical one inch wide road bike tire requires two to three the psi than a wider mountain bike tire to support the same amount of weight. Any Physics majors out there who can explain this in greater detail?
Actually, that's not completely the case. Lower profile tires generally have less lateral air capacity, so a higher air pressure and/or a stronger sidewall is needed to retain tire shape as exterior pressure is applied. It's not that low-profile tires need higher pressures; it's that low-profile tires have less rubber to deform when exterior pressure is applied -- by weight from above, a bump from below, etc. One way to mitigate that is to design the tire to operate at both a higher overall PSI and a narrower range of PSI -- which is part of the reason why road-cycle tires operate at relatively high PSI compared to off-road cycle tires. (Another reason: off-road tires simply have more rubber available to deform.)

Boyle's Law: As exterior pressure is applied to a fixed volume of gas, the gas's pressure increases because the volume cannot be increased. This is why significantly higher air pressure is needed in trailer tires under load than unloaded: the weight compresses the rubber of the tires so that a higher air pressure is needed to maintain the tire's section profile.
 
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