Originally Posted by
livedarklions
I suspect your tire would be flat the next day. As we know from party balloons, the small helium atoms escape through the pores of the rubber much faster than air molecules.
It certainly would diffuse a lot quicker than air, maybe not completely flat by the next day, but noticeably softer is my guess.
On the other hand, you could save some 15 g of rotational mass!
(I recently did a little calculation on this just out of interest; don't remember the exact figures, but it was something like that. For mountain bike tires it would be even more, over 20g!)