Since the current LX platform Chrysler 300 along with the Dodge Magnum and the later Charger debuted in 2004, it has had an optional all wheel drive system. Much of the platform hardware of the LX was derived from the previous generation Mercedes-Benz E-Class including that all wheel drive hardware. The AWD system was designed to continuously drive all four wheels all the time and it can be a boon to those who live northern climes. However it comes at a price that includes reduced fuel economy. Driving the front axle all the time costs about 1 mpg compared to the rear wheel drive version. For the 2009 model year, a revised system will now disengage the front axle until excessive slip at the rear axle triggers drive torque to sent to the front. As a result, AWD models will now get the same fuel efficiency rating as the RWD variants.
[Source: Detroit Free Press]