An other gas which outperforms nitrogen and can be used for aircraft tires is sulfuric hexafluoride, i.e. SF6. The molecule is huge compared to nitrogen and leakage rate is extremely low. An other big advantage of SF6 is the excellent extinguishing performance in case of fire.. .However, since SF6 is a global warming gas, its use will be quite limited in the future.. .SF6-gas is also used in gas-insulated switchgear for electrical high voltage insulation applications and in the sport shoe markets, e.g. nike air cushions are filled with SF6.
Cheers
CB