Drivers are complaining that their real-world fuel economy doesn't
match up with the official estimate given by the Environmental
Protection Agency (EPA)...

So why doesn't the car or truck you drive to and from work every day
get the same fuel economy the government, the manufacturer, and us
trusted legions of automotive journalists seem to promise it will?...

The answer is as simple as it is ultimately complex. New vehicles, as
tested for their energy consumption, are never actually driven
anywhere, much less to and from work, and their fuel economy ratings
are not ultimately based on how much fuel they consume...

http://infotruck.blogspot.com/2005/0...will-vary.html