Tyre pressure

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The owner's manual for my Hyundai says that if you drive at altitude the tyre pressure should be increased, by 10kPa/km elevation. The way I look at it, it's the pressure difference between the tyre and the atmosphere that supports the car, so the pressure should be reduced (though you'd have go to quite a height to make it worth the trouble).
What does anybody else think?
 
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Ummm... is this an answer looking for question?

So, IMHO if at altitude you were inflating a tyre from completely flat (i.e. newly fitted) then I can see the benefit as the ambient pressure, as rightly surmised, will be somewhat lower so by inflating to a slightly higher pressure (1.5lb/psi per 3000ft) will compensate when returning to sea level. Also it is possible, unlikely, but possible when checking tyre pressures that they will read slightly higher at altitude when checking.
That difference will probably be lost in the error of the various gauges.
 
40 years ago when I was working in a VW/Audi/Porsche dealership in Cape Town, we had to retune cars for altitude that came down from Jo'burg. Ignition timing and carb mixture I seem to remember.

My Yamaha FJR1300 would adjust such things automatically for altitude riding.
 
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so by inflating to a slightly higher pressure (1.5lb/psi per 3000ft) will compensate when returning to sea level.
I doubt that's what they have in mind, I think they mean pressure needed at the altitude.

But something I hadn't thought of - most likely the airline pressure indicator reads gauge pressure i.e. relative to the actual ambient atmospheric pressure. In that case, for a given tyre pressure a gauge reads higher at altitude (lower pressure on the atmosphere side) and reducing it to the specified sea-level figure gives the correct (by my reasoning) pressure difference between tyre and atmosphere.
 
My guess is that it's to take account of the reduced "squeeze" of the atmosphere on the side walls. Because it's less at altitude then the tire has less force applied to the sides and so provides less cushion to the wheel.

Or I'm other words, external air pressure and tyre pressure are additive not reducing.
 
The owner's manual for my Hyundai says that if you drive at altitude the tyre pressure should be increased, by 10kPa/km elevation.
At what elevation is that pressure measured, if measured at the altitude then the correction will ensure when at sea level the tyre pressure is still correct, so I would guess it is talking about when up the top of the mountain, not before you start the ascent.
 
The owner's manual for my Hyundai says that if you drive at altitude the tyre pressure should be increased, by 10kPa/km elevation. The way I look at it, it's the pressure difference between the tyre and the atmosphere that supports the car, so the pressure should be reduced (though you'd have go to quite a height to make it worth the trouble).
What does anybody else think?
It is because the gauge itself reads higher because of reduced atmospheric pressure at altitude; not that it is necessary to more inflate the tyre when you get to the altitude.

The actual pressure in the tyre remaining the same because the tyre is strong enough to not expand with the reduced atmospheric pressure.
Imagine a metal gas bottle compared to a balloon.
 
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