the epa sticker estimates of expected gas milage are based on the automotive equivalent of a global (warming) climate model, that has reliability somewhere between pukey and amc gremlin.
Yeah, they could come up with a real mileage test. You set up a test track with turns, hills, stop lights, freeway, etc. Have the acceleration of the car controlled by computer to simulate the way an "average" driver hits the gas. You could even have a computer drive the car around the course as the typical driver would. You could also have an old lady mode and a kamikaze mode and rate the car on all three.
Of course, it won't happen because the numbers will plummet.