5.5 Reduce speed by 50%, OR

6. Some combination of 4,5,6.

This is kindof dubious. If one takes one's foot off the accelerator you car will roll to a stop in under a minute. Ergo, rolling resistance and aerodynamic drag are dominant factors in fuel consumption. If they didn't exist, one could just accelerate to speed, put it into neutral, then shut the engine off and coast the rest of the way. Reduction in kinetic energy via reduction in speed alone just isn't so important IMO. One must reduce vehicle weight, thereby reducing BOTH kinetic energy AND rolling resistance simultaneously. And besides, it already takes a while to get to where we are going. If we commute 30 minutes to work every day, nobody wants to stretch that to an hour. It would be a troll.

Of course, motor vehicle manufacturers want to sell bigger and bigger vehicles because they cost more. They increase their cash flow, and profit margins. People also feel safer and more powerful in big vehicles. Now that this has been done, to reverse the trend might be very difficult. People do not want to drive a less powerful, less safe vehicle compared to others on the road. But if they don't they will eventually be forced to give up driving completely due to high fuel prices. Financing vehicles with monthly payments makes the cost of fuel much more obvious to drivers. Instead of a car costing $20,000 and the fuel costing $1000, now people are paying $250 per month for their car and $120 per month for fuel. The market is much more sensitive in the near term, and motor vehicle manufacturers are heavily invested in gas guzzling models. Suddenly demand for gas guzzling vehicles drops, manufacturers lose their customer base and can't switch tooling fast enough to turn out models that people might want. Bankruptcy.