+1. Nitrogen still permeates out so you have to check anyhow, even if you don't suspect a leak. 

I get 3.74 grams/liter for Krypton at standard temperature and pressure so roughly around 20 grams in a tire at 65 psi, or maybe $6 a tire for a Krypton fill. Still way too expensive for me. 

I can't wrap my head around all this talk about different gases.......I'll use a good gauge, check my tires and enjoy the view.   And hopefully not fall in the water without a spare tire.

And that is at standard temperature and pressure. Add a factor of 3 for 55psi and you just spent $960 to fill your two tires. I think - Dissenters please feel free to post your homework -

KN

Could be the price - If my rusty math serves me, filling with Krypton would  run around $120 per tire - the stuff is roughly $33.00 per hundred grams. If you'd like to try your own math skills use the formula for volume of a torus - Post your results - I could be wrong.

  Warning - my math is not that reliable.

KN

You'll need a better fridge than your Dometic which barely keeps food cool. The freezing point of Krypton is about -250F. 

BTW - Congrats on getting your tires sorted out, in spite all of our editorial comments. Enjoy your trip!

Now, an old tractor tire, being a much lower pressure tire, would not only hold a whole lot more air but  would reach steady state at a much higher % O2. If its maintained at 10 psi it would end up at about 14% O2. All ya gotta do is find a way to let only a little air out at a time so you don't become that boated corpse.

Diver: "I repeat, a huge bloated corpse with its face attached to an old 400 pound tractor tire and wheel"....

Oh lord. 

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). 

 

Then it's unlikely they are nitrogen filled. 

SO...carry on with normal procedures.

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.

Many long haul trucks use nitrogen inflated tires (at least on the steer tires) because of this property alone. The large fleet I drove for had very good success with nitrogen filled tires. 

True, but my advice stands. Here is why:

IF the tires are factory-filled or dealer-filled with nitrogen, they are much less likely to have been under-filled, or to have both lost 10 psi just sitting around. Nitrogen filled tires hold proper psi for 3 to 6 times longer than regular air filled tires. 

If they are filled with 'regular' air, then slow air loss (over a few months) is considered by all of us to be more or less, normal, so two 65 psi tires sitting at 55 psi would not be unusual, meaning the gauge is probably correct.   

Still, it is very little trouble to verify a new gauge against a known, accurate, gauge. 

In most gas stations these days, I wouldn't feel to confident about the accuracy of the gauges on the hoses since they are dropped, battered, and left out in the elements to get crud in them.  You can buy a nice dial tire gauge for <$20, but if you're going to spend that money, you may as well buy it with the compressor attached for a few bucks more.

If you need to drive slowly a few blocks, or a mile or two, at slow to normal, urban speeds, you can still consider the tires 'cold' (more or less)...But if you have to drive 5-10 miles in hot weather at highway speeds to some distant location just to check and add air, then the tires are no longer 'cold'.

If your r-pod is new it might have nitrogen filled tires. Mine does. You can add regular air if you need to, but if your tires have green valve stem caps, and/or a nitrogen sticker on the side of the pod, and BOTH of your tires are at 55psi, you need to check your gauge against a known good quality gauge, or have the tires checked at a tire shop to be sure. They may be at 65psi and your gauge could be incorrect. 

Have you verified the accuracy of your tire pressure gauge?