For the automotive minded:
I know that the engine load decreases when I place my transmission in neutral, so I would conclude that the engine is using less gas. However, does it make any sense to shift to neutral for short periods (less than a minute or two), such as waiting at red lights? Or would any potential savings in fuel and air pollution be offset by added wear on the transmission? OTOH, would the transmission possibly benefit from spending less time under load?
This could be an urban legend but believe it or not I’ve heard of there being laws against this (putting your car in neutral at a red light). The logic being that if for some reason you had to suddenly react to something you might forget to put it back into drive first…
That seems odd - honestly, I can’t imagine a circumstance where suddenly having to accelerate from a dead stop quickly would possibly be necessary.
In theory, putting your car in Nuetral at a light would save a little gas.
The Prius actually turns off to save gas at stops.
As far as Hail Ants:
I know in NJ they mentioned in Drivers Ed that it was consider unsafe by the state to coast down a hill in Nuetral but they never said anything about at a stop. Many people with Manual transmissions do both anyway.
Coasting in neutral with an automatic may prevent it from lubricating properly.
Putting it in neutral takes the load (which is minimal) off the engine, but in most cars the idle speed will be higher in neutral, offsetting any savings. The difference, if any, is so picayune I doubt anyone’s even bothered to make a study of it.
The transmission proper isn’t affected by this. The load is resistance to fluid flow through the torque converter, which at idle speed won’t cause any measurable wear.
It’s really not worth the bother to be mindful of when to shift nor the effort to move the lever. Any time a driver has another task to do it distracts a little from his awareness of what’s going on around him, and any time a car does something unexpected – like not pulling away from a stop in good time because the driver neglected to put it back in gear – it increases the chance of meeting some other car whose driver didn’t expect it to be there. I’d say it’s more of a safety issue (small, I’ll grant) than a fuel savings issue (negligible, at best).
I’ll bet it costs more. Give anybody the chance to rev their engine and they always wind up revving their engine. In a real world test at the hands of the masses, too many people would rev their engines in nuetral.
On paper, a tiny am’t of fuel would be saved per car, adding up to alot over time.
In the real world: vroooom…vroooooooom…vroom…vroom vroom…
Not everybody. As a counterexample, me.
My geriatric car tends to heat up when moving slowly or stopped in gear for long periods. If I come up to what I know is a long light (getting to the light as it turns red, say) or I’m in heavy traffic that grinds to a halt, I put the transmission in neutral. I don’t rev the engine. Never have, never will. What’s the point?
While they don’t actually address the question of gas usage, the Car Talk guys have something to say about this topic:
I sit at lights in neutral because i don’t like holding in the clutch. Also, my dad (a long time mechanic) told me it accelerated wear on the throwout bearing.
But as for descending hills in neutral…that’s just dumb. Without engine compression, you will accelarate down a grade, and will be forced to keep braking to keep your speed down.
Also, most modern fuel injected cars incorporate some type of deceleration fuel cutoff. The injectors are stopped when there is a reverse load on the engine. Going down a hill totally off throttle will cause the fuel consumption to go to zero. Since the wheels can keep the engine turning, it doesn’t need fuel at that time.
However, if you descend in neutral, the engine no longer gets turned for free, and the car has to use a little fuel to keep the engine at idle. It’s a small amount, but more than none.
So descedning in neutral only serves to waste gas and wear out brakes faster. Great idea!
Be that as it may, cruising on my New Haven/DC trips my oil temperature sits in one range. When traffic locks up, or just crawls, it quickly climbs into another range entirely. Not quite in the danger zone, but I’m pretty sure it would get there eventually. Dropping into neutral when I’m going to be stopped for a longish period of time helps keep the temperature down.
Your car probably has an electric fan that kicks in when the temp goes up to run air across the radiator for this very reason. Apparently it kicks in at a higher temp than you do. 
Please don’t get the impression I do so or recommend it. I even stated (but you snipped w/o indication) that
to do so.
Please indicate you are taking a snip of a quote if you are going to be that negative in a response.
Sorry if it sounded like I was saying that you endorsed it. It was really a monologue directed at nobody in particular. I didn’t call you out.
Just a bit of ranting rhetoric based in fact, that’s all. Don’t be paranoid, man 
Its ok, I got the feeling you weren’t ranting against me
It’s just good to include the …snip… so someone else glancing though doesn’t blast me, its happened before and will happen again.
*(Of course this was talking Politics or Nuclear Power not Trannies, so your comment on being paranoid strikes very true) *
I was taught when driving that it is safer to put your car in neutral when stopped. The theory is that if someone crashes into the back of you you could take your foot of the clutch and the car would shoot forward into the back of another car or pedestrians crossing the road. Admittedly this was in a manual, but I’m sure the same argument can be made for an automatic.
Automatics really aren’t designed engineering-wise to be continuously “manually” engaged and disengaged as part of normal driving. The Car Talk guys are right re it putting additional unecessary wear stress on the drive train.
Correction -
Automatics really aren’t designed engineering-wise to be continuously “manually” engaged and disengaged as part of normal driving. The Car Talk guys are right re it putting additional unecessary wear stress on the internal transmission components