The Union of Concerned Scientists have two articles in its Clean Vehicles series: Fuel Economy: The Single Most Effective Step for Cutting Oil Dependence , and Ethanol: Frequently Asked Questions which are well worth reading and generally support my views as to what we need to do to reduce our dependence on oil imports. They have put some numbers on the scenario that they develop.
Their conclusion is: "The importance of fuel economy is thereby twofold. First, it delivers bigger savings in the near to medium term. Second, it makes it conceivable that biofuels could almost completely replace gasoline for our cars and trucks in the long term because we would need much less fuel."
If our average fuel economy is increased to 40 mpg in 10 years, we have some modest fuel economy improvements beyond that date, and also add in some policies to help reduce miles traveled, all of the oil in the blue area can be saved. The green area represents the potential savings from ethanol, if everything goes well in growing that industry. In the near term our saving from biofuels is modest, but by the end of the period biofuels nearly replace all gasoline usage. Projected demand would drop to the line between the blue and the green.
Their concern over the automotive industry is: "Over the past 20 years, automakers have used advancements in technology to add more than 800 pounds to the average vehicle and nearly double horsepower, while fuel economy has been allowed to slip. Today we have ample technology to preserve or improve current size, utility, performance, and safety characteristics, while increasing fuel economy to 40 mpg within 10 years. And over the next 20 years, hybrid technology can deliver even greater gains in fuel economy. This will provide the groundwork for us to make effective use of alternative fuels in the future, and will give us time to sort out some of the challenges associated with a shift to alternative fuels. ... The savings from better fuel economy would keep on growing indefinitely, while the oil wells would dry up."
Not mentioning plug-ins or all electric vehicles is a glaring omission, but to be kind their last sentence could be interpreted to include them.
They state that the long-term potential for corn grain ethanol is fairly limited—that we currently use about 10 percent of our corn crop to displace less than three percent of our gasoline needs—but that it is a key part of transitioning to cellulosic ethanol. The biofuels they consider are a combination of cellulosic ethanol and Fischer-Tropsch fuels. This scenario assumes that the yield per acre of cellulosic feedstocks is doubled and that the the gallons-per-ton yield from feedstocks of biofuels is more than doubled.
From what I have read their assumptions on yields per acre and gallons-per-ton are very attainable. Their article on ethanol provides a good overview on what we can expect from ethanol and provides some insight to their expectations from ethanol.