Honestly, this is how I see it playing out:
This 8% or whatever of our fuel useage will be cut out with the new standards. Assuming that manufactures can achieve it...Well, on second though, let's assume a conservative 5%.
Now add a conservative 30% useage of cellulosic/corn Ethanol in this country (there's a current cap of ~30-40% of our vehiclular demands can be met by ethanol.
Factor in a liberal amount of Volt-esque vehicles(smaller cars, and Crossovers); 40%
Then for fun, let's do 10% of our fuel useage being cut out by the Hybrid cars everybody is having fun with
That's
very roughly 85% percent of our vehicle fuel useage NOT on fossil fuels within 20 years as I see it.......
Now, how fast we can do this is another question. China and India aren't slowing down their growth......