Not sure I fully agree with TR4A's comments about freezing at altitude, the thickness of the tyre wall would prevent the onset of any freezing inside the tyre. The tyre would have to cold soak for many many hours to have any appreciable effect on the inside temperature of the tyre, this can be confirmed by a glance at the tyre pressure page on the EICAS/ECAM during a long haul flight, the temperature ( and pressure ) drop is marginal. Many manuals allow a tyre to be filled with air as a ' get you home ' measure until the tyre can be purged and refilled with N2.