They typically cover 25 miles per hunt on the garmin, and have yet to show any signs of being too hot. Even in 90+ temps and mile+ races.
Not trying to sound like a hater, but I suspect that the Garmin is not getting reset everytime you open the dog box door. I would tend to think that may include some travel distance while they are in the truck. I've been wrong before though. I won't lie, my dogs could not go 25 miles full out in the temps we're having now. There would be some form of damage that was visible to me if they tried it. I'm sure of that.
Cutter, I can assure you also that the garmin is reset every time... several folks on here have hunted behind them. How do they do it... I don't know. Here's a couple of pics of the mother/daughter in question and a son/grandson who I believe is going to be built just like them when he's finished growing.
I would think the black dogs wouldn't have the heat tolerance. I guess the very thin hair, thin skin and loose skin has something to do with it.