Originally Posted by Camping Couple
Nitrogen is an inert gas. It will not expand or contract. If you need to add air, go for it. At this point you should keep an eye on your pressures as adding air will now affect the the pressures with heat.
Of course it will expand and contract! Just like air - which as pointed out is 78% nitrogen! Being an inert gas means it normally doesn't react with things, as opposed to say oxygen, which tries to combine with almost anything!
There is absolutely NO reason to use nitrogen in normal consumer vehicles, and mixing it and regular air makes no difference. The only good thing to say about using it, is that it does no harm.