Why the advertised MPG is so BS, and real world MPG is much less.

http://www.fueleconomy.gov/feg/how_tested.shtml

Doesn't give the accurate details of how they are tested by the EPA, AAA gave these details of the old test method, that has been in use since the 70's. The last time the test methods were revised was 1985.

This test presumes the speed limit is 55 (not for 10 years of more), that air conditioning won't be used, that the vehicle will take 18 seconds to get up to 60 mph.

Automakers under a new law will not be forced to display the more accurate (and worse) MPG estimate until the 2008 models.

AAA says that the smaller "city" mpg is closest to the true real world average mpg that your vehicle gets regardless of all other factors.

Starting in model year 2008, estimates will reflect the effects of:
Faster Speeds & Acceleration
Air Conditioner Use
Colder Outside Temperatures
http://www.fueleconomy.gov/feg/ratings2008.shtml

Once the new methods are in place, the EPA expects mpg estimates to drop for city driving by 10–20 percent from the original estimate for cars and trucks and by 5–15 percent for highway driving.

Hybrid vehicles will also be affected because the nature of their technology makes their fuel economy more sensitive to factors such as cold weather and AC use. Hybrid city mpg estimates are predicted to drop 20–30 percent while highway mpg estimates may fall about the same percent as that for conventional cars.

http://www.naftc.wvu.edu/NAFTC%20eNews/January%2006/epatest.html