Tire Loosing Air

[Andrée-Anne Bourgeois]

Not that much for sure... In fact, you have it almost the opposite.

The formula for perfect gases is (in French) : PV=nRT

P is pressure in kPa
V is volume in liter
n is the quantity of gas in mole
R is the perfect gases constant
T is temperature in Kelvin

So, for a fixed volume and quantity of gases, pressure change is directly
proportional to temperature change (temps are evaluated in absolute Kelvin
only). 

Then, if the temp drops by 10F between 68F (293K) and 58F (288K) (that would
be about 5 Kelvin, more or less), that would lead to a drop in pressure of
about 1% (293/288).

So, if you filled your tires in October at 50F (283K) and now temp is 0F
(255K), pressure would drop by 10% (255/283= 0.90).

On a car with 32 psi, that's a loss of 3 psi.


So a crude rule of thumb could be 3 psi per 50F or 1 psi per 15F.

Louis-Alain



-----Message d'origine-----

Isn't there a formula, for every degree below freezing, that a tire looses
10 psi?