Why not use Helium and reduce the unsprung weight and being an inert or Noble gas Helium will not corrode the wheels or react with the rubber of the tire. Argon would work too but it is heavier than Nitrogen.
Not to mention the fact that helium leaks through almost everything. Got a rubber helium-filled balloon? Notice how it deflates itself over only a day or two? It's diffusing through the rubber. Newer helium ballons are made of mylar (or something like that) so they won't leak down by diffusion through the container.
Actually, filling a tire with pressurized vacuum is best. No mass at all. I keep a couple of cans of vacuum from Prestone around just in case I get a flat.
The pre-packaged cans are expensive, though. I'm thinking of getting a vacuum compressor from Checker Auto. Seventy-five bucks, last I looked.
Terry, 230RN
