Believe it or not, one of the biggest hurdles to more fuel efficient cars is highway safety standards. Each safety design feature adds additional weight that reduces fuel efficiency by increasing the inertia each engine must overcome.

As far as pneumatics goes, that is actually one of the oldest automobile technologies. Before the internal combustion engine was vogue, steam engines powered "horseless carriages." Leave it to the French to repackage 150 year-old technology into a "new" idea. It's also not efficient.

Here's a link to an article that sums up my thoughts pretty well.

NY Times Link