Lutz
You know an awful lot more about these things than I do - I find your comments consistently helpful.
The point I was making was not so much in the context of the 85% rule but the overall additional weight in the context of how well the tow car performs.
The landcruiser, Discovery and the Kia are heavier and more powerful cars than my RaV so will almost certainly pull a Challenger better without causing undue strain on the actual car.
I used to have a Shogun Petrol (LWB. I hated it, but it did pull the caravan brilliantly without any worrying drop in fuel consumption (it drank like a fish anyway)
So, all I am suggesting is that provided the right kind of car is picked, the easier it will be to tow the Challenger, without having to worry about what you have in the car etc.