I found these interesting nuggets of info in
"Automotive Electrical Systems Circa 2005
By John G. Kassakian, Hans Christoph Wolf, John M. Miller, and Charles J. Hurton
Demands for Better Fuel Economy and more Electric Power are Driving Cars to Multiple Higher Voltages"
IEEE Spectrum: Automotive Electrical Systems Circa 2005
"Even more to the point, the U.S. Corporate Average Fleet Efficiency (CAFE) standard prescribes a maximum fuel consumption rate for cars sold in the United States. The Federal government assesses a penalty of US $5 for every 0.1 mile per gallon (0.04 kilometer per liter) below 27.5 mi/gal (8.55 L/100 km) on every car the manufacturer sells. A 200-W electrical load accounts for about 0.4 km/L in the FTP 75 cycle test; so, if a manufacturer is delivering 25-mi/gal (9.41 L/100 km) cars, for example, it can justify spending more per vehicle on components to improve electrical efficiency."
Also:
"Improving the electrical system's efficiency so it lops 100 W off the average electrical load has the same effect on fuel economy as reducing the car's weight by 50 kg, as measured by the FTP (Federal Test Procedure) 75 standard profile of starts, runs, and stops"
and the real kicker:
"At present, when U.S. and European fuel economy tests are conducted, only those electric loads essential to the operation of the vehicle are active—that is, the ignition and engine electronics. Lights, air-circulating blowers for the passenger compartment, entertainment electronics, power windows, and so forth are all turned off for the tests."
That last one has changed slightly in that now they must run the A/C for a short part of the test but still no other electrical loads are required.
Anyone still not clear as to why new cars often don't live up to their mpg ratings.