Can it be done?  Can we make cars that get 50 mpg?  Well, of course we can; after all, just look at the space program.  Except that the space program has a large budget.  It doesn't have to produce a final product which provides a compelling cost/benefit ratio to consumers.

I'm sure we can make cars that get great mileage.  However, if it could be done with existing technology, at a price point that consumers would be willing to pay, the market forces would already have the automakers doing it.  They aren't, which tells me that we can't.  At least not yet.