This may sound stupid but I'm wondering how do you actually get the right tyre pressures. I've always just checked the tyres when cold and pump to the 29 front and 36 rear recommended pressures, which is what the manual instructs. However, when checking the pressures on my Saab today, I noticed that Saab effectively defines 'cold' as 20C and goes on to say that a variation of 10 degrees in air temperature will affect the pressure by about 1-2 psi. Now to be perfectly honest I've no idea whether they're implying I should under or over inflate by 2 psi or so, given the average temp is 5C at the moment. Is the variation in temperature between winter and summer really worth taking into account, as my Saab manual seems to suggest. [&:]
Tyre pressure question
- Thread starter jw2015
- Start date
penfold944
New member
Funnily enough, I used to have a saab about 2 years ago and I found that then I pumped the tyres to the recommended pressures that the car was incredibly skittish. I think Saab were saying 36 in front and rear - but the car was terrible to drive at those pressures.
I find with the boxster that the recommended pressures are fine - the hardest part is finding an air supply at a petrol station that is even vaguely accurate !
The only time I've adjusted the boxster pressure is on track days when the tyres warm up. Pressure increases at higher temps, so I would guess drive the car for a few miles and then check the pressure.
Or I could be completely wrong !
I find with the boxster that the recommended pressures are fine - the hardest part is finding an air supply at a petrol station that is even vaguely accurate !
The only time I've adjusted the boxster pressure is on track days when the tyres warm up. Pressure increases at higher temps, so I would guess drive the car for a few miles and then check the pressure.
Or I could be completely wrong !
Guest
New member
I think the way it's written is misleading.
The tyres need to have a certain stiffness. This is produced by 29 Front and 36 rear. It doesn't matter if the temperature is minus 3 or plus 40 degrees. Porsche still recommend that your tyres are 29 and 36 PSI.
The second part of the paragraph tells you that as the temperature changes you need to keep an eye on your PSI's. As it warms the gas tries to expand and the pressure goes up, therefore making your tyres too rigid and too bouncy. Basically keep checking you PSI's as the temperature changes.
Just in case I was talking rubbish I checked with my wife. She's an anaesthetist and what she doesn't know about gas isn't worth knowing. At least that's what she tells me.
The tyres need to have a certain stiffness. This is produced by 29 Front and 36 rear. It doesn't matter if the temperature is minus 3 or plus 40 degrees. Porsche still recommend that your tyres are 29 and 36 PSI.
The second part of the paragraph tells you that as the temperature changes you need to keep an eye on your PSI's. As it warms the gas tries to expand and the pressure goes up, therefore making your tyres too rigid and too bouncy. Basically keep checking you PSI's as the temperature changes.
Just in case I was talking rubbish I checked with my wife. She's an anaesthetist and what she doesn't know about gas isn't worth knowing. At least that's what she tells me.
The way I see it, if I check pressures at say 0C ambient temp, then I should be putting in 3-4 psi less than the temp recommended in their tables, which are based on 20C ambient temp. If I pump to full recommended pressure at say 0C then surely I'll be 3 or 4 psi too high when the tyres heat up. I'm not sure why Porsche don't seem to worry about it. It seems that the guys who track their cars check pressures constantly and maintain the same pressure regardless of temp, but surely the manual should recommend checking pressures on warm tyres rather than cold then. The more I think about it the more confused I get.
Guest
New member
No, I'm afraid that's not correct. You should have in the recommended PSI whatever the temperature and watch the PSI as it warms or cools. It's only when the temperature changes by 10 degrees centigrade that the pressure changes by 2-3 PSI. Belwo this level you'll get a 1-1.5 PSI change which is the tolerance of your average tyre pressure guage.
So fill in winter and let some out when the average temp is 10 degrees higher.
The way you are running things your tyre pressure is a couple of PSI too low for six months of the year.
So fill in winter and let some out when the average temp is 10 degrees higher.
The way you are running things your tyre pressure is a couple of PSI too low for six months of the year.
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