This is my opinion, Don't but a new car... instead lease it for either 2 or 3 years. The reason I say this is that I believe the car manufactures will come out with MPG around 50 or better, with this you would have a hard time getting rid of a car you buy today. If you lease it you turn it in and go for a different one. The government is calling for better fuel mileage not matter if we want it or not.
Ford already has a car they manufacture over seas that gets 65 mpg. It is a diesel and Ford has decided that they could not make money on this car in the US as of today, but none the less the technology is already here.