Yah, and you know the auto manufacturers are 'holding back' because it would be awful if one of them developed a technology that was light-years ahead of the others. They're probably all in collusion, too.
</sarcasm>
Seriously, auto manufacturing is among the most cut-throat businesses on earth - if there was a chance ANY major auto manufacturer could push the efficiency of their vehicles significantly, don't fool yourself into thinking they'd sit on it because electric and hybrid cars give them the ability to 'slack'.
Fact is, we had gas powered cars in the 80's that are more efficient than most cars on the road today. While the modern internal combustion engine is remarkably efficient, we're building safer and safer (read: heavier and heavier) cars today while trying to push the MPG up. Very tough equation. If you study the efficiency of the modern engine, it's actually quite remarkable.
Sure, there can be improvements, but if you think the auto manufacturers are sleeping on tech because they don't 'have to use it' to meet standards, I'd say you're way off base - if any of them had some tech that allowed a significant improvement in fuel efficiency (that held the same standards wrt reliability, etc) you can believe they would be implementing it ASAP.