I keep a log book in all my cars. Every time I fill up, change oil, anything I enter it. Every fill up I calculate fuel mileage. My commute is 89 miles one way. 20 miles of winding, hilly farm to market road, 49 miles of wide, fast two lane road with a few stops, and 20 miles of wide, nice freeway. It is very rare indeed to see my mileage fall below 26 MPG for my commute.
There is something additional to some of the efficiency comments above, that I don't think most people would think about. I'm a stick shift driver. There are several places where I come to a stop sign at highway speed. I have my points where I simply put it in neutral and coast a ways before stopping for the sign. I have noticed that my MB's will coast MUCH further than my '88 stick shift Vette. If drivetrain and tire friction were the same on these cars, the Vette should coast further and faster. It has much less frontal area and decent aerodynamics. I believe that this is due to less drivetrain friction, imbalance and other mechanical losses from the poorly engineered GM POS. The MB's are a much better engineered and produced piece of machinery.
This is my opinion from my highly unscientific research.
Have a great day,