Cylinder deactivation is an automatic switch from six cylinders to three when the load is light. This means that, for example, on long, flat highway drives, the engine will not burn as much gasoline. The cylinder deactivation feature will see first use in the 3.9L V6 engines used in the 2007 Chevy Impala, whereas the 3.9L V6 with E85 capability will only be available in the Chevy Uplander fleet models. Chevy announced that "Preliminary testing of the 2007 Chevy Impala equipped with the 3.9L V-6 with AFM indicates an estimated 20 mpg in the city and 29 mpg on the highway – improvements of approximately 5.5 percent and 7.5 percent, respectively."
[Source: World Car Fans]