This is a huge technological challenge, but I'm sure not impossible for the smart and determined --
Given the state of the economy, energy, and the environment, if only engineers could come up with a way to retrofit existing car engines to make them vastly more efficient (while maintaining reliability and safety).
The challenge of course is to come up with something that could easily be done to massive numbers of cars, without a lot of individual tinkering, and which doesn't reduce reliability and safety.
It would be easier for people to afford retrofits than whole new cars, it would reduce our energy usage a lot faster, it would more readily stimulate the auto sector than waiting for the economy to rebound enough for the whole-car industry to rebound, and it just might be a bridge lifeline for the car industry (especially parts manufacturers).
I have two cars that I own outright, they are relatively late model, and I am not in a position to replace them. I have been irked from day one that when we needed to replace our old cars, there were no energy efficient cars in good safety classes.
If Volvo or Toyota or Ford (or ?) came up with an energy efficient safe car tomorrow, I couldn't afford it, but I would be willing to spend a few thousand to significantly upgrade the energy efficiency of my existing cars, if I knew I could do it fairly easily (without it becoming some overwhelming personal project) and the results would be reliable. I think a lot of people would.
It would be a perfect project for one of those towns in the midwest where everyone is out of work all of a sudden because of the auto industry down turn...