From my understanding, for fuel injection engine, going down hill with transmission in "D", fuel is injected into cylinder as long as key is in "on" position, in fact it should always operate at or above the floor.
But it consumes less fuel than normal driving condition, because without the need of extra throttle opening, the computer reduces the duty cycle of the fuel injectors, less fuel is injected into cylinder.
Put into "N" will save negligible amount of gas, but you will lose some maneuverability, sometimes you need to use brake to keep speed down, it may overheat brake system, all these actually increase the possibility of accident.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.