2008 Window Sticker Mileage Plummets.
Beginning in 2008 new car window stickers will reflect more realistic fuel mileage figures. This will result in a drop of 12% to 30% from prior years! Fuel-saving gasoline electric hybrids will see the biggest drops in the mileage numbers posted on their window stickers.  According to the EPA, as much as 30% worse in city driving, 20% worse on the highway.  
 
Changes to the 30 year-old government tests:
Temperature:  The new tests start with the car at 20 degrees Fahrenheit. The old tests started at 75 degrees. A vehicle uses more fuel during warm-up. Cold temperatures are especially hard on the batteries that power hybrids, which is partly why those vehicles take such a big hit in the new ratings.

Speed:  The new tests use a maximum speed of 80 mph instead of 60 mph in the old test.

Acceleration:  The new test includes hard acceleration. The old test used gentle acceleration that simulated 0-60 mph in 19 seconds. That's another reason hybrids suffer. Hard acceleration requires full use of the gasoline engine and little or no participation by the fuel-saving electric engine.

Air conditioning:  The new test assumes it is used 13% of the time (and that still seems low). The old test didn't use air conditioning at all.  

New car owners have complained for years their cars wouldn’t get the mileage indicated on their window sticker. There’s a reason for this; the old tests never reflected real-world driving. 
Source - Wall Street Journal