I've been away from a computer for a while.
Robert, I'm very sorry if my posting caused you distress. I'm also very sorry about any injuries, and damage to your vehicle.
However, a couple of points:
1) The speed rating of 15.5/80R20 XLs is 50 mph, not 55. The thermal stress on a tire goes up as the square of the speed. At 61 mph the thermal stress is ~48.8%> than at 50 mph.
2) My stock U500 tops out on a level road at 70mph indicated (speedometer), but only 68 on the GPS. 16.00R20 XZLs are exactly 12.5% larger in rolling circumference (397 rev/mile vs. 447) compared to 395/85R20 XZLs. One can calculate that with 16.00s a U500 could go 76.5 mph; not 86. How does your U500 go 9.5 mph faster? You must have also been reading off a GPS, the speedometer only goes up to 75.
3) When was the last time you weighed the vehicle? Expedition trucks are like people, they gain weight as they age. My rear axle load gained 1100 kg over 2 years.
4) It's not at all clear from your comments how you knew that the tire only had 30 psi in it when it read 65 psi, since it and its' axle-mate were probably quite flat after the accident
Thank you for your warning about the beadlocks. I am aware of that possibility. I tried mounting one at a truck tire shop in Tucson and when they finally got it installed they couldn't fill it presumably due to inlet hole obstruction. When I got home I drilled 1" holes over the inlet fitting. But my local truck tire shop couldn't even get them installed. With that in mind (the impossibility of working on a tire in the field with beadlocks) I decided to go "bare" and not go below 22 psi in front and ~35 in the rear.
Lastly, I have derived a formula for calculating tire temperature from pressure change (assuming the indicated pressure is correct!):
Thot = Tcold X (Phot + 1 bar)/(Pcold + 1 bar). Temperatures are on absolute scale. 1 bar is added because a "flat" tire has 1 bar in it. This is of course only valid near sea level. That this means is that on a 80 deg morning in the desert, a 95 psi cold tire reaches 114 psi at 180 F.
Again, my apologies for any distress I caused you.
Charlie