I have a 2017 F-250 gas engine, and a BP horse trailer that, when fully loaded, should be 6500 - 7000 pounds. I understand that in order to save my brakes, I need to "downshift" when going down a hill, but I don't know whether this means I should change from D to 1, or to 2, or if I should use M and try to shift manually? Or if I'm using tow/haul mode will the truck do it for me?

 

You did not mention what kind of tranny you have. I would use the brakes. That's what they are made for. Brakes are cheaper and easier to fix instead of stressing the transmission/ clutch and revving the heck out of the engine. Now if you are going down Pike's Peak, that is a different story.

 

Something to consider,
picture this , you have placed the trans in a lower gear to assist with downhill speed control AND you have a heavy trailer behind you pushing you downhill. If that trailer is not controlled, but the vehicle is trying to slow down, you will create a jacknife condition and can lose control of the entire combination.

What do you think about this?

 

That was what I had originally thought, but over and over again, from professional truckers, from people who like me haul horses, from people who criss-cross the country with travel trailers, I'm hearing that I need to downshift. I can't ignore the weight of opinion.

Also FYI I will be hauling my horses across the Rocky Mountains, so I am definitely expecting some significant downhills.