I'll believe some of those fantastic mpg claims when I see them. It's all about physics, guys; there's no free lunch. It takes a given amount of energy to produce a given amount of work. You can tinker around the fringe, but there's always a price to pay, or a lot of compromise. Sure, you can lean em out, lighten them up and shrink the size of the engines, but in the end you have to choose your level of comfort and driveability.
Anyone who thinks that the auto manufacturers are holding back on the technology to increase efficiency hasn't looked under the hood of some of the new models. My daughter just bought a new Chevy Cruse that has this dinky little 1.4 liter engine which is unbelievably high tech; it's even turbo charged. It is supposed to get about 38 mpg highway, but I wouldn't bet on it--at least not with any passengers and not in the mountains. There is a price too; the sheet metal the body is made of is so thin that her front fender now has a dent from a soccer ball stike. I can only imagine what a minor hail storm would do to it, or hitting a tweety bird.