I know this is contributing to thread creep...but late (ie high speed) descents do not save fuel!
Even at idle a jet engine requires fuel to keep it lit. This is directly related to the mass of air flowing through the engine. More speed = more air = more fuel. The fuel saved by extending the cruise, typically 20 to 30nm is outweighed by the extra fuel burned in the high speed "idle" descent. In the 737 it's not unusual to burn an extra 100 to 200kgs in a high speed descent. The most economical descent is to trade time for fuel and descend early and slow, which is exactly what the FMC does when you change the CI.