Here are some comments about that subject from a car forum (not me)
--------------------------------------------------------------------------------
I think they may have been inhaling something other than helium (
) ...maybe you are thinking of nitrogen, that is used in some aviation applications (it doesn't leak out as easily as air and is more stable). But it's generally not worth the hassle for automobiles.
As far as improving gas mileage, 30 psi in the tires is 30 psi regardless of the gas used to inflate it so there's no advantage there.
As for helium's lifting power, that only happens if the helium is uncompressed and allowed to expand so that it is less dense than the air that surrounds it. When you pump it into a tire to 30 psi it is under heavy compression and likely heavier than air (that's why compressed cylinders of helium don't float off the ground).