thanks for the explaination waldo, and steve but i still dont get it.

as waldo stated, helium, and nitrogen are both less dense then air, so how would you inflate the tire? would you (rough calculation) double the maximum pressure of air? would the tire be able to hold 64psi of gas?

and another thing, both helium and nitrogen are highly flammable? this seems very dangerous to me, both for personal saftey and to the car if say someone dropped a lite cigg. next to a tire with a slow leak?