I Know this wall of Text will not get you interested, but it will help save you, and your car.
While your car is moving, if you change your car from D to N will hurt your Gear box and the break pad. seriously.
How it hurt the gear box: Imagine the D is like the bicycle rear gear(act as car gear box) spinning with the chain(act as wheels), when u change it to N, is like taking the chain out from the gear while it is moving. it will hurt the gear and eventually make it not "makan" anymore.
Changing from D to N while moving will also numb your car, the situation is after you change it to N, the engine is not controlling your car, so the wheels is spinning on its own, and if the speed of all the 4 wheels is irregular, this will make the car out of control and eventually roll the car (if it slide side way) since you don't have anti roll bar in the car to prevent it from rolling
Just say like this, now, all 4 wheels is running on the same speed (eg. 60kmh) then u change it to N. if one of the wheels(front right wheels) drop into a potholes, that very wheels will slow down and this situation ( 3 wheels spinning on 60kmh and 1 on 50kmh) and the car will start to lost balance. This is because when you are in N gear, the WHEELS are powering the ENGINE(which u CANNOT control) instead of the ENGINE Powering the WHEELS (which u CAN control).
How it effect Breaking: when a Car Break, actually it is not just the break pads breaking your car, the engine is ALSO breaking at the same time.
LET SAY IT THIS WAY : [while on free gear/N, you depend 100% on brake to brake. same if you are in manual gear but press clutch. if you are engaged in any gear, engine will also help braking. because the engine decrease it running power ]
ANOTHER MORE ADVANCE EXPLAINING: [D to N and back to D on downhill is bad, it's called 'shift shock', your gearbox is spinning faster and faster as it goes downhill, while the drive from the engine remains at idle and suddenly you force both to match up again which is where the shock happens. this means, the wheels is powering your engine and it is a not very good sign actually]
If you think i bullshit about the ENGINE BREAKING, look here (Neutral/No gear (N) section): http://en.wikipedia.org/wiki/Automatic_transmission
ENGINE BREAKING is something like the engine will decrease it gear (like gear 4 -> 3 -> gear 2) before breaking to decrease the inertia from the gear with the engine. waja won't feel it because it is an old car/maybe you can't feel it because is no experience about other car. These old car's engine will need to detect a certain rpm before lowering it gear automatically.
explanation 2: the engine is lowering it gear to slow down the car. it is just like a manual car running from gear 4(100kmh max) to gear 3(60kmh max) to gear 2(40kmh max) to gear 1(20kmh max). So, it is easier to break.
Conclusion on D-N while moving : N gear, the breaking pad will take all the burden on breaking. D gear, the break is breaking the car speed, and engine is also breaking at the same time.
Changing from D to N before breaking is NOT SAVING YOUR FUEL AT ALL.
Proof about not saving fuel while using N gear at downhill and eat up your break pads : http://www.popularmechanics.com/cars/how-to/4230705
Overall Conclusion 1 : Change from D to N before breaking = engine sakit, transmission sakit and break sakit = engine, transmission and break spoil easily = shorter half life = need money to change because it spoil faster
Overall Conclusion 2: In an AUTOMATIC GEAR BOX, the only gear you can change when your car is moving is just L, L2, D2, D3, D4, D (or whatever your car manufacture name their low gear)
Overall Conclusion 3 : The Physic syllabus we learnt SUCKS! no teach me about engine breaking.
Overall Conclusion 4: Actually, not applying acceleration in D, or using engine braking, ast higher revs, doesn't use much gas at all. The engine is being made to run by the wheels at that point, rather than burning gas.
It saves more gas to engine brake to a stop than to slip into neutral and use the brakes alone.
actually there is more technical name.. not using cause you guys might not understand..
P/S: If my physic essay was this, i think i get A+ already. type by my own self and a little help from SIFU at LYN FnF section. i might got some missunderstanding sentences, cause i'm not good at explaning stuff.
P/P/S: just advising you not to change from D to N while moving, seriously. Is not like it is saving fuel at all. try to get know how the fuel jet in your engine work and you will understand. the only way to stop fuel flowing from fuel tank into engine is turning off the engine.
If you change from D to N while moving, it is like killing the vehicle slowly, it might take more than few years, it might take less than few years, depends on luck.
to safe fuel. don't use sport rims, pump up and check your tires every month, don't accelerate too fast.
use smaller tires? lol.... cause bigger tire will result in larger rims(heavier) and increase rolling resistance and the more fuel the car used to accelerate cause need more energy to spin the heavy wheels that got a lot of resistance on rolling. :)