[citation][nom]InvalidError[/nom]You cannot make a self-powered air-car since the air-compression/decompression process is very inefficient. On top of that, you have mechanical losses due to friction, rolling losses in the tires/suspension, aerodynamic losses from drag, more losses from braking, etc. so no matter how efficient you manage to make everything, you still need some form of external energy source to top off the compressed air tank.[/citation]
^ This.
Everybody reads about these and thinks "Ohh, it runs on air -- it MUST be earth friendly!" The thing is, the air is not the energy source; it's the energy storage. The energy needed to compress the air is still coming from the same old filthy electicity sources we've been using all along. (Those are the things that really need revolutionizing.) To top it off, air compressors are among the more inefficient electrical devices, so this is actually quite wasteful.