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This is regarding a 2007 9-3 Aero sedan with 235/45/17 tires.

According to the SAAB 9-3 owners manual, tire pressure should be 35 PSI.

So, in cold weather, pressure is obviously going to fall. And when you drive the car (which warms the tires), pressure will rise. According to the manual, a 10-degree rise in temperature (Fahrenheit or centigrade?) will result in a pressure increase of 2PSI. Unfortunately, the manual doesn't go on to tell us how much tires typically warm up during average drives.

So, to what pressure do you inflate the tires in cold weather, i.e. before you have warmed them up by driving them?

If you inflate them to 35PSI then go driving, pressures will obviously go above the recommended 35PSI. So clearly I will have to inflate them to less than 35PSI. But how much less?
 

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You inflate tires cold, fluctuations from heat of moving are nothing to worry about.

You should check tire pressures at least monthly (who does?), but I guess that means that if you live somewhere that has significant seasonal temp variations you should check and inflate / deflate to address the temp variance at cold.
 

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Pressures given in the handbook (and at garage forecourts etc) are always standardised at something like 15 degrees celsius, and you should always adjust the pressure when the tyres are cold. That said, you can always measure the pressure before you set off, and then again when you get to the service station. Whatever the difference is, that's the adjustment you need to make against the book figure...
 

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Yes pressures should be set cold. Variation will occur during driving, and will be different for each tire. (ie: the tire next to the hot muffler will heat up more than the one on the other side. Front tires more b/c engine heat, etc). The pressure increase during use is taken into account when standard pressures are chosen.

You are correct that for every 10 degress ambient temp change, tire pressure changes 1-2 degrees.
 

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Set it to 35PSI cold. A few PSI above the recommended 35 will not adversely affect the tire performance. Better high than low IMO.
 

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I'm currently checking my pressures almost every trip, thanks to a slow puncture!!! :evil: :evil: :evil:
 

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Pressure rises with temperature. I estimated how much after seeing a road bike tire to explode in sun. You can do an estimation with simple gas equations (p*V/ T = constant)

Even your temprature change was from freezing 32 F to 100 F, the pressure changes only about 11 %. This is not very much for car tires. In road bikes, where you may have more than 100 psi, this is little more significant.
 
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