People who claim 'they could build a 400mpg Hybrid' amuse me.
 

Not so much because of the statement, but because they don't have any
math to back-up said statement. Might as well say, "If I flap my arms
real hard, I could fly." Yeah. Sure. If it WERE possible to build a
400MPG civic or Prius, Honda or Toyota or Ford would would already be
doing it (and bragging about it on the television).



There are limits the universe places on Energy-to-Motion conversion.
You can't exceed those limits. That means NO civic hybrid or prius
hybrid will ever get 400 mpg. As it stands now, both these hybrids
are already operating at 40% efficiency. Even if you increased that
to 60% (a miracle), you'd still only boost them from 50 to 60mpg.

And if you're a wizard like Gandalf who can magically boost a Prius
engine to 100%, such that you had a perfect 1-to-1 conversion w/o
losses, that's still only 125mpg.

In other words, "a 400mpg prius" is not only a bad idea.
It violates the Laws of the universe.
It's perpetual motion.
Impossible.

Now if you leave behind the "standard" car shape, and try something
exotic life a Jetsons-car, then you might be able to do better.
Volkswagen did exactly that a few years ago, with a 1-seat car, 8
horsepower engine, and shaped like a cigar.

But even then, they still only got 250 miles per gallon.