well,some shift to N-mostly to step off the brakes in traffic...though an automatic vehicle should basically be on D...N is rare, probably when towed

@BS

Think of it like this, when does a manual car consume more - with a gear engaged or free? The same case applies to automatics. When on position D (Drive) the gear is engaged. The engine is loaded, its only the brake or terrain that can stop the vehicle from moving. On the other hand when in N (neutral). The engine runs freely no load. Tell me when should it consume more?

Hint : when the revs are high (model dependant)when you car is idling, it means either there is a problem somewhere, its deliberately set at that point or the engine has not warmed to its optimum temp of operation.

In traffic I usually have it on N and engage the hand brake. To rest my foot. Fuel savings as @mukiha puts it is insignificant. Turn off the engine if stationary for long - that way you know you are saving fuel.

As for the RPM - possibly once a gear is engaged, there's a set RPM for that gear and thus could be a lower RPM than when idle. When idle for a petrol engine, typically should be an avg of 800 rpm. Less on a diesel I think (don't own one).

It is definitely debatable... but the saving is insignificant. What I would find even more debatable, is whether its a good thing or not.

http://www.google.co.uk/...mp;biw=1280&bih=909

But look for 'knowledgable' contributors to these debates.

As for you RPM rising it happens in some cars.There may be nothing wrong with your car. Many mercs will show the rpm dial rising slightly when you go into go into neutral compared to when in D and foot on brake.... nothing wrong with the car.

If at N it's revving higher it means your fuel injection or cooling system setting is very high and is wasting fuel. Also check whether you have enough engine oil and engine coolant and that your A/C controls work. When a car is idling it should burn the least because it only needs enough power to continue running the engine and a few components working as opposed to power to keep the engine running and transmission. Check your coolant and that you have enough oil, if no solution check the system settings at a qualified mech workshop!