Cars, etc.

[Bob Zumbrunnen]

LOL!

Initially I couldn't tell if you were pulling our collective leg or he was pulling yours.

It's the latter.

A pound of pressure per month is quite normal. As you said, oxygen molecules are quite small and can escape a seemingly non-porous tyre.

A lot of track folks use nitrogen to fill their tires. Doesn't leak through the molecule-sized pores as easily and has stronger thermal momentum. Hot tires stay hot longer and don't go through the hot/cold cycles as rapidly and frequently as air-filled tires.

Temperature variations (both ambient and tire temperatures) have a major impact on air pressure, as well. Depending on how aggressively you drive and how much air pressure was in the tires when cold, the difference between hot and cold pressures can be several PSI. The softer the tires when cold, the greater the increase in pressure when hot, due to the heat generated by the softer tire flexing more.

It's for this reason that it's under-inflated tires that blow out; not over-inflated ones. The huge increase in pressure from all the flexing combined with the weakening of the tire from the excess heat.