The New York Times > Business > Average U.S. Car Is Tipping Scales at 4,000 Pounds:

"Average fuel economy peaked at 22.1 miles to the gallon in the late 1980's, according to the agency, but has eroded since then to 20.7 miles for the 2003 model year."

Why? I imagine that this even takes into consideration the new hybrids. How discouraging.