Well folks, no more hiding behind illusions. The EPA has updated the MPG estimates that all vehicles sold in the US are required to display on their sticker. The good news is that these estimates are more realistic, because they account for things like lower fuel economy in cold weather. They also account for things like air conditioner usage, higher speeds on roads (check it out: the site shows an image of an 80 MPH speed limit sign) and faster acceleration — things that have changed since the standards were introduced in, probably 1980 or so?
I can say that the estimates are quite accurate for my car, the Toyota Prius, still the best mileage of any car. They look pretty accurate, if dismal, for most of the rest of the US fleet of cars.