How about filling your tires with Helium? A floating car would get far better gas mileage. Just stick some sails on it, and your good to go!
(yes that was a joke)
=)
Understanding that “helium” was a joke, it would also be a poor choice from the standpoint of diffusion, eg “leaks”. Want a bigger molecule. The argon choice posted elsewhere on this thread would be fine.
It's no goofier than the con game of filling 'em with nitrogen.