You still haven't explained why you think N->D will destroy an automatic transmission, nor why you think you're in neutral instead of merely having the torque converter unlocked.

The behavior you observed, regardless of its actual nature, is not universal (nor did you suggest that it is). Here's one data point: I tested in my 2002 GMC, and the torque converter does not unlock at 65mph when I use the "coast" cruise control function, when cruise is on while going downhill, or when I merely take my foot off the gas. At tbat speed, the only conditions that will cause the torque converter to unlock are a throttle-induced downshift (whether from cruise or my foot), or me manually selecting "N".

Just because the torque converter is unlocked, it does not mean that power is not being transferred from the engine to the transmission or vise versa. It wasn't until the late 70s or early 80s I believe where you saw vehicles being equipped with torque converter lockup. If anything, having the torque converter UNLOCKED would mean higher RPMs, not lower. The torque converter locks up at steady speeds, not when accelerating, and or if accelerating, at a moderate pace. More modern vehicles today support not only full torque converter lockup, but some even partial lockup at half way and then some where the torque converter lockup is variable much like a CVT transmission's gear ratios are to an extent.

So, you're saying that while you're decelerating downhill, that if the TC was unlocked, the engine would rev up faster???? mmmmkay.

I have a feeling you're confused about the purpose of a torque converter and when the torque converter is unlocked and should be locked. In Older vehicles since the torque converter was unable to lockup, downshifting and going down a hill would result in weak engine braking. But in newer vehicles, supposedly the torque converter is suppose to lockup to result in BETTER engine braking.

In simplest terms, think of a torque converter like a clutch, it allows slippage between the engine and the drivetrain. It's not a perfect example but it's good enough for the purpose of this discussion I feel.

Nope. totally clear on what a TC is thanks.

So you understand that if the TC is unlocked while going downhill, with a closed throttle the revs would drop?

Good so now we can get back to...

How do you know it's going into neutral and not just unlocking the TC?

Actually I don't, I really really have absolutely nothing concrete to prove this, it's really just my intuition. But it is my understanding that a torque converter that is slipping the most that it can would still yield higher engine RPMS than having the transmission in neutral. And when I held the cruise control downward in order for it to "reduce speed" the RPMS dropped to what I believe is idle for this car, effectively being in neutral.

If it's not shifting into neutral and simply just telling the torque converter to slip" or even to completely disconnect which I'd say is basically neutral, then what we should be asking ourselves is this:

What would put more stress on an automatic transmission? Shifting it into neutral while moving and then shifting it back into Drive, something the manual tells us NOT to do. Or when attempting to glide, telling the cruise control to "decelerate" which we speculate either shifts the transmission into neutral for us, in a controlled manner and or causes the torque converter to slip and or entirely disengage.

Modern toyotas seem to have a funny way of defining "idle speed". When I coasting, I shifted the car into neutral and even though it indicated I was in neutral, the "idle speed' was around 1000RPM despite the engine being fully warmed up. Also getting the car to be at its lowest idle speed has been quite a challenge and for the most part I've found it to idle around 1000rpm but sometimes 800rpm even when fully warmed up.

I confirm that you have only moderate knowledge of how your automatic transmission works *************. an automatic does NOT have to shift to neutral between gears. being in only one gear at a time is solely an issue of manual gearboxes, regular or sequential. an automatic uses 1-3 planetary gearsets with clutch packs and brake bands to control gears, all that's required to shift gears in an automatic is to engage or disengage. if it's in X gear and wants to go to the next gear and it's in the same gearset, all it may have to do is release a brake band and viola it's in the next gear. the power drop between shifts is slippage of the clutch packs themselves with low line pressure (which most cars, especially GM products, have). If you do like me and play with the throttle position tranny cable (miscalled the kickdown cable usually) my toyota A340LE transmission would engage with enough pressure to accelerate during upshifts from the raised line pressure as the engine revs were brought down.

they also do NOT shift to neutral when coasting or stopping. the automakers and insurance companies decided that having engine braking automatically combined with DFCO was the best trade off of safety/FE. all you're seeing is torque converter slippage (you also need to look up how these work) which on most cars is around 2k rpm difference. My toyota A340 stall speed with the 7MGE is 2100 rpm. that means if you hit the brakes and floor it, the engine rill rev as high as 2100 rpm without moving the car. why is this relevant? it means that by design, the engine speed can be +/-2100 rpm from what it would be if the torque converter clutch were locked up in a given gear.

be careful making sweeping statements like "shifting from N-D WILL destroy your transmission". It won't usually. maybe if you have crappy gearboxes from the start. I have a buddy that's been beating his Toyota A340 gearbox for 70k miles with 170k on the car. that's frequent redlining, manual holding gears to redline (with both normal and tweaked line pressure), manual shifting, and redline neutral drops (that's launching from a stop by flooring it then going from neutral to drive) N-D =bad is an insurance/CYA (cover your ***) thing. it usually doesn't do any harm especially if you give it a little gas to rev match. (the torque converter make it VERY forgiving).

ALL cars have a very funny way of defining idle speed... generally you have a cold-idle speed to warm up, hot idle speed when not moving which is the lowest you'll get it, and moving hot idle speed which is a little higher to facilitate N-D shifts and also partially because most cars have a cold-air snorkel to somewhere on the front of the car, which is gonna have higher pressure if only half a psi.

There are some many falsehoods in this thread based on "intuition" that's its embarrassing. An automatic does not select gears the same way a manual does. Yeah. I thought this thread would have information useful to help me with P&G, but it seems I will continue with my own avenue of experimentation.

Well considering a lot of vehicles aren't meant to be dinghy towed combined with shifting constantly from gear to neutral and back again while still moving sounds like a disaster waiting to happen with driver's manuals stating this. Also I doubt many people would be perfectly rev matching since that would mean more work and less benefit to mileage.

That's a dumb way at looking at what you've read. Just because the theory behind the idea is incorrect, it doesn't make the idea itself any less valid. Furthermore how the transmission shifts is completely irrelevant to my discovery because when gliding, the tach maintains a constant speed, as if it was in neutral which goes back to what my "intuition was".