Auto transmissions: Does a car in park or neutral use less fuel than in drive?

Factual Questions
1

Question in title. Vroom vroom!

2

First off, I assume you mean when it’s not moving.

I would think so since it’s not wasting energy pushing against the brakes.
Having said that, for a multitude of reasons, I wouldn’t suggest changing gears at red lights.

3

yes.

4

I mean, yes, when stopped at lights.

Why not switch to neutral when stopped?

5

Wear and tear on the transmission, extra time to switch gears when the light turns, can’t react as quickly to get out of the way if the need arises. I suppose it’s moot if you’re gridlocked, but if you’re first or last in the line, you wouldn’t want to. Hell, even if you don’t have a car on the left or right (meaning if you really needed to you could still get out) you might as well leave the opportunity open.

As for wear and tear, I’ll let someone with more knowledge of the inner workings of an AT talk about that, I’m really just making an assumption on that one.

6

The simplest automatic transmissions use planetary gear sets which are capable of creating several gear ratios in a small package.

Great animation here.

Different parts of the gear set are held in place to create different gear ratios. Usually a friction band will be the mechanism that holds certain parts still or locked to the transmission case. Multi-disc clutches do this as well but they also hold different parts of the transmission to spin together like the input and output shaft*. When you move from N to D you slide the manual valve which allows hydraulic pressure from the transmission pump to apply either one of the friction bands or multi-disc clutches. This would not cause wear if both parts that are being held together were not moving or at least not moving relative to each other. When your wheels are not moving but your engine is running there is a relative motion between the input shaft and output shaft in Neutral but will be held together in Drive. From this it is simple to see that going from N to D does indeed wear your transmission more. If it is enough to significantly shorten the life of your transmission, I have no idea.
*Although the input and output shaft are spinning together that does not mean that they are spinning at the same RPM. The difference in RPM, if there is any, depends on what gear you are in.