What makes tires lose pressure: time or miles?

[George Bynum]

Originally Posted By: otis24
For every 10 degree drop in outside temperature, you lose one pound of tire pressure. I've heard it stated that filling the tires with nitrogen will negate that.
The 1 psi per 10 degrees F is a pretty good approximation. Pure nitrogen, the 93% or so stuff used to fill tires, or the 78% nitrogen/21% oxygen we breathe in the atmosphere all behave the same ... P1V1/T1=P2V2/T2, the "ideal" gas law simplification applies to most gasses.

Whether or not "nitrogen" inflation has ANY advantage with automobile use is addressed often in many groups. I believe I'm part of the vast informed majority who feel that almost all of its supposed advantage is that it tends to be dry.

 

[gathermewool]

Originally Posted By: otis24
For every 10 degree drop in outside temperature, you lose one pound of tire pressure. I've heard it stated that filling the tires with nitrogen will negate that.



Nope - expect approximately the same change.

What some of us experience is actual leakage. Temperature is irrelevant if I last add when it was cold and now have to add again when it's warm out.

 

[CapriRacer]

My experience is that pressure is lost both through the tire and around the sealing areas - the rim and valve hole - and about equally.

So while time is a factor, so is the amount of miles driven as flexing the tire very slightly moves the lower sidewall area relative to the rim.

It's recommended that pressures be checked once a month, but my experience (and the experience of others) has been that this is too often, unless you have an active leak. It's also said that normal leakage is on the order of a psi or 2 in a month.

 

[TiredTrucker]

For comparison, my heavy commercial truck tires, I check the air pressures a minimum of once per week, and more often with seasonal temp changes. Those tires run 2500-3000 miles a week, and rarely change a pound of pressure in that time. Seems average temps play more into it than anything else.