This just isn't the way of the future. We are getting as good as we can with gasoline engines. Diesel, hybrid, plug in, etc. are all better ideas than this. You want a 25% improvement in gas mileage, switch to a diesel. You want sustainable, run it off soybean oil or waste cooking oil.
I've got a full size sedan with a big engine that's ok for my rare long car trip (like on vacation). I'll keep that, or maybe get a smaller conventional car for that purpose, but how many households have a second car (my wife and I have been carpooling to work for the past six or seven years, but that's not going to last much longer). We'll need a second car.
I want a nuclear powered car. Hold on...my commute is about five miles. I run an errand on the way home, maybe got to dinner, say 15 miles per day. Give me a plug in electric car that's good for 40 or 50 miles on a nights charge, and charge it off a nuke heavy electric grid. That's no problem from an engineering standpoint. Maybe something that will give me 80 miles on an eight hour charge? If they can't do that they will be able to soon. It would be a second car, not my household's only car. I have the dinosaur powered car for trips, and my wife can drive it to work. If every two car household did that, and we build nuclear power plants, we could greatly decrease our oil needs.