First, if you will call your local Trooper... you will probably find it is ILLEGAL....
And it is not safe for the reasons mentioned above...
and it MAY not be providing the proper lube at the speed you are talking about....
Remember that an auto trans still has the same innards as a manual ... it is only the SHIFTING which is done ' automatically'.. and driving something from the back end where the power is usually applied to the gears in the other direction is not a safe game...
WHY TAKE A CHANCE ON MESSING UP A PROPERLY WORKING TRANS ? Have you checked on what it takes to rebuild on of these ?

A gasoline car burns MORE fuel coasting in neutral than it does coasting in gear. Modern computer controlled gasoline engines (i.e. all of them) shut off fuel delivery completely when coasting with the engine RPM high enough - they burn zero fuel. Highway speeds would always qualify. By shifting to neutral you force the engine to burn fuel to stay running when you could instead use gravity and the kinetic energy of the vehicle instead. The result is increased fuel consumption and increased pollution.

Also, dropping the transmission in and out of drive at highway speed is extremely hard on the transmission and the entire driveline of the vehicle. When you shift from N to D, the trans does not simply drop into overdrive - it goes for first gear (second on some Mercedes). It then shifts up as rapidly as it can. I've never been in a vehicle which didn't show some nasty signs of distress when this was done - a big drag and lurch while the electronics and trans tried to figure out what the hell was going on.

Best practice is to just leave it in gear; it'll delivery better fuel economy and less stress on the driveline.

- JimY

Coasting in neutral with an AT will eventually cause the trans to burn out. I would change the filter and fluid and NEVER do it again.

After thinking about coasting in neutral at high speed.
The driveshaft turning at 55 mph., engine turning at 700 rpm.
There probably would be some friction.
Interesting question though.
When one set of clutch packs are engage, aren't the other clutch
pack disks rotating, until the next gear (range) is selected?

I know on most cars, coasting caused some pressure inside the trans and will blow seals in the trans, causing severe leaking.
Thats one thing nobody has pointed out, so far. Good Luck....

I thought MTs have parallel shafts each with a variety of gears that mesh while ATs have a sun gear, planet gears and an array of clutches in a configuration I've yet to understand.

Don't torque converters have a stall feature so very little power is transmitted in P and N?

Sixto
95 S420
87 300SDL
83 300SD