I don't know, but that is when I started to back off. I wasn't doing anything radical, I was just towing through some slight hills (near FT Worth on I35) at a reasonable highway speed (70-75), with an empty trailer (~6000 lbs). I was running stock since I had just got my truck back from the dealer before the trip, and didn't have time to reinstall my goodies. I did have my dashdaq though. That is how I found out that it was getting hotter than I really liked. The coolant temp was getting to 228* too, and I found out that the fan kicks on at 224* every time.
I wasn't doing anything that Joe Schmoe Ford owner wouldn't do if they didn't have any clue about such things. I figured I was completely safe, since I was running stock. I figured that Ford would have designed the computer to not allow any operation that would harm the truck. And it didn't, but I am wondering what the upper limits of safety are when it comes to temperatures. I know that it will go into limp mode during severe overheating. I don't know what severe over heating in terms of actual temperatures though. I didn't have a real coolant temp gauge on my F450 when it did that, so I don't know.