tire pressure

I don't go around testing old tires for nitrogen/oxygen ratio, but I would assume it's not 80:20 inside the tire after many years.

So, the permeability is proportional to the difference in the partial pressures of each gas between the inside and outside of the tire.  If the permeability of N2 through rubber is about 1/4 that of O2 and there is 4x as much N2 as O2 in the normal atmosphere, the two ratios roughly cancel, so the total amount of N2 and O2 lost are about the same to start with. 

So if you start with 80 psi of air (65 psi on your gauge because that is measuring delta pressure relative to an atmospheric pressure of 15 psi)  that will be 64 psi N2 and 16 psi O2. After some period of time you might have 59 psi N2 and 11 psi O2 for a 16% O2 mixture which would give you 70 psi  (55 on your gauge).  You'd never get to 0 percent O2 because the loss rate of O2 would get slower and slower relative to the N2 loss rate as time went on.  If you air your tires back up to 80 psi (65 gauge) then you can continue the process of slowly increasing the N2 content of your tires over time, but you'd still never get to 100% N2.

Still, if you were trapped underwater with another person you didn't know with no source of oxygen other than two tires, one old and one new, you should grab the newer tire. Unless of course it was nitrogen filled, in which case you'd be totally screwed).