Hopefully some of this information will help out some folks

Heres my setup since I tow quite a bit, mine is a little over kill for what most people would ever need:
Scan Gauge II
Aux Trans Cooler
Aux fan on cooler for stop and go traffic:
Rear Airlift Air Bags

I also have an OBA setup now with rear airbags to avoid the sagging but you dont need those if you dont tow often

I'm aware of the 220 ceiling which is why I'm trying to get people to post what temps they get on their SG. Do you have any idea where the SG temp is read? Before the cooler? After? Inside? Why wouldn't the "ATF HOT" go on when my SG reads 257 degrees? What is that set at?

I'm in CO and run over a lot of high passes with my trailer which is why I'm looking for info. Short spikes in the temps is not my concern, running temps is though.

The ATF light is a dummy light, Im not sure what its set at, but by the time that light comes on its too late. When your hitting those high temps are you in 5th gear or OD? Mine will start to climb pretty quickly when I have it in OD and I hit any type of hills or if the torque converter has to unlock. When that happens I down shift it to 3rd and the temps cool of very quickly. Of course the RPMS are pretty high at about 3500 but I would rather waste fuel than burn up the transmission.

Like you said, you shouldn't be overly concerned with very short term spikes but if the spikes are that high I would be concerned IMO. From my research most people see around 180-220. Do you have a trans cooler yet?

I am wondering where the SGII pulls the tranny temp from? If 220 is the theoretical safety ceiling, and the dummy light gets activated at 250 then how can CXS (Chris) see 257 with no dummy light? Chris are you running the OEM tranny cooler that came in your v8?

Allow me to share my little knowledge from my occupation.

It is very hard/difficult to determine which temp sensor Toyota uses for the main temp reading. Such as which one does the dash light use and also the SG. I know the ECM uses both for varying reasons.
The idiot light is programmed to come on at 240. It should be pretty accurate but I can't verify that and am not willing to push my fluid that high to see!

The highest I have seen on my SG is 202. That was during a 90 degree day while four wheeling in some very slow and aggressive trails. In this situation the engine compartment gets very hot and with little air being pulled across the trans cooler it is to be expected.

While on the road I have noticed that they temp will start rising without fail if I do not keep it in 5th gear/OD. At a stop light if you shift into N it will start climbing as well. This is because the pump is not moving the fluid as rapidly.

Ambient temps have a huge difference on my trans temps. Yes, this is pretty common sense but it REALLY makes a bigger difference than I would have imagined. I am assuming it's because the cooler is the most forward and gets most of the air flow. Remember that at idle it is just the opposite. The engine fan air flow has to pull air from 1st the radiator, then condenser and finally the trans cooler. If your cooling fins on any of the 3 are restricted by dirt, leaves, bugs etc it makes it that much harder.

In the HVAC business they use a product for cleaning the condenser coils outside of the house or business. I have some and use it on my condensers and also my vehicles. This stuff is absolutely amazing. Especially if you use it on your home condensers you will freak when you see what this stuff cleans out. I bought a gallon jug of it a few years ago and can't remember where I got it. HD and Lowes sells it in a aerosol can. I like the fact I can put it in a spray bottle. Here is a link from Amazon.
Nu-Calgon 4291-08 Nu-Brite Coil Cleaner - 1 Gallon, Blue: Ac Condenser Cleaner: Amazon.com: Industrial & Scientific

One more thing. If you ever think or know that your fluid is getting dangerously hot and you pull over do NOT ever turn the car off. All the manuals will tell you to pull over and shift to park and allow it to cool down while idling. However, I would leave it in D with my foot on the brake. As I stated above my temp always rises after I shift to P from running pretty hard.

Ya'll have posted some good ideas and thoughts. I figured I would share mine as well.

CSX/Chris~If I ever am able to determine exactly which sensor they use you will be the 1st to know! These 2 sensors are very close to each other internally so it's not like they are on opposite ends of the system from each other.

This is interesting. I know for a fact the FSM states 240.

I will hook up my Techstream today and do a live data run on all the running conditions and post the readings back here. Although it is a pretty cool day air in Atlanta.

I was going off of memory. If the FSM says 240 that sounds better.

I wasn't trying to argue your statement at all. For some reason that 240 sticks in my head as I was reading that FSM word for word trying to determine how this temp crap works.

I am running the stock cooler which I think I'll upgrade before next season. My confusion stems from the SG reading vs. FSM info stated in this thread. It makes me think the SC is displaying a higher than actual number. I keep mine in 4th gear, no OD and the ambient temp makes a huge difference which surprised me.

This is what I found regarding temps.

The acceptable operating temperature for automatic transmission fluid is between 175 and 225° F.

As the temps rise to 240° F additives in the fluid begin to cook and form varnish inside the transmission.

Around 260° F the internal transmission seals begin to harden resulting in leaks because the seals lose their elasticity.

Around 295° F the clutch plates begin to slip as the oil continues to break down.

Over 300° F the seals and clutches begin to burn out as carbon forms in the oil and the transmission fails.

I got too busy with a bunch of minor things getting ready for my trip out west. I will run some tests tomorrow while out and about.

Oh no, no worries. I appreciate you checking. I was going off memory.

Another question, how does altitude impact tranny temps? I'm at 5K feet and my SG reads around 180 around town with ambient temps at 80.

My tranny also reads 180 around town; 3' elevation. It reads 140 on the highway.

IIRC someone earlier in this thread mentioned much lower but looking back I see it was @mccoyle who has a pic showing 82 which got me wondering. Now I'm guessing it was a cold start and photo.

Considering the 5,274' difference between our locations that I can rule out altitude as a factor.