Side air bags will make those 3 cylinder 100 MPG plastic cars safer than the 8 cylinder 20 MPG behemoths.
Or you can just get a 3 ton electric car. 1 ton is enough car to carry you, your kids, and the groceries, 1 ton is enough batteries to power that car, and 1 ton more car is enough car to carry the batteries. So, 3 tons.
And punt that SUV into oncoming traffic.

3tons is 6,000lbs. The new crossovers weigh in at 5,000 and that already is unacceptable.
Also not many people buy vehicles purely for fuel economy, which is why we're in the predicament we're in!
As for plastic, it failed already in the marketplace cuz it doesn't hold its shape well, its heavier than sheet metal and as oil goes up, guess what? it becomes much more expensive.
All the FMVSS requirements add weight to vehicles.