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.
I reset my Garmin every time I hunt. Last hunt....sunday a week ago. Hunted for 2 hours and the dogs had been 20 miles. Time before that it was 17-18 miles. It's not too far fetched if the dogs are working and tracking. But yes...a fraction of that is time in the ranger from one turn out to the next.
