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Correct tyre pressure for car in cold weather

I am in the UK. My car is specified to have a tyre pressure of 26 psi front and 29 psi back.

I had pumped up the tyres in warmer weather when the air temp was about 18 to 20 C. I would probably have driven about a mile or more to the filling station to pump up the tyres so by then the tyres would be warmer.

Today the air temperature is about 3 C. I took a tyre pressure reading (using my usual digital tyre pressure meter) after the car has been standing for over 24 hours.

All the pressure readings today seemed a bit low. They were about 4 to 5 psi lower than specified. i can't think that all four tyres had leaked by a similar amount.

Is this pressure difference mainly due to the effect of temperature? I ISTR something like Charles Law allows this to be calculated although there seem to be some other variables too.

Can anyone advise?

Thank you.

Reply to
Zak
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PV=nRT; in a tire, sorry, tyre, V, n, and R are constant unless you add air, which changes n.

The pressure specification is for in use, not the parked condition, so drive the car around a bit then check the pressure.

Reply to
jimp

And remember T = temperature in Kelvin, not deg C, ie add 273 and a bit.

cheers, clive

Reply to
Clive George

The message from snipped-for-privacy@specsol-spam-sux.com contains these words:

All the handbooks I've ever seen have specified the pressure as the /cold/ pressure.

Reply to
Guy King

Just drug the handbook out of one of the cars; it does say cold pressure.

But, it goes on to say:

"Cold" means your car has been sitting for at least 3 hours or driven not more than 1 mile.

So, if it is in the middle of a freezing winter and the car has been sitting out overnight, I would suggest once or twice around the block before checking the pressure would be appropriate.

Reply to
jimp

Buy a Hummer. Tire pressure is controllable in real time.

Reply to
Uncle Al

So the percentage change in pressure is...

Assume: Summer = 20C Winter = 0C

% = (273+0)/(273+20) * 100 = 93%

If you measure 29 psi at 20C in the summer you should find they measure about 27 psi in the winter at 0C.

If the tyres were hot - say 40C when you measured them in summer then expect

25 in winter.

Drive around and remeasure.

Reply to
CWatters

Why?

Reply to
Gary McClean

Why not?

In retrospect, it probably won't make much difference unless the car is totally cold soaked, the tires, umm, tyres are filled, and one goes off for a long, high speed drive, in which case the tyres would be slightly over inflated.

Reply to
jimp

Tyre pressures specified in manuals and manufacturers info, are for cold tyres...

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"...Be sure to check the pressure while the tyres are cold, and have not been used recently. If you drive even a mile this will cause your tyre pressure to increase and give you an inaccurate reading."

G.

Reply to
Gary McClean

What about the slight expansion in volume of the tyre as the pressure increased?

Could that be significant?

I would hazard a rough guess that it might be up to 5%.

Reply to
Bill Woods

Yes, I have seen the same. But a given pressure taken when 'cold' at

-5 C is not going to give the same pressure while driving at a constant 75 mph as the same given pressure taken when 'cold' at 25 C.

IYSWIM!

I guess the type will probably warm up to the same temperature when it is in use irrespective of a 30 degree C difference in air temperature. If this hunch is correct then the amount of inflation does depend on the actual value of the 'cold' temperature when the reading was taken.

Reply to
Bill Woods

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