My gasser has a 5 Speed Allison, in my case the MODE switch is labeled "Grade Brake".. if yours is then post and we will describe its operation to you. otherwise..
I mostly put it in "D" and let it do it's job.. I've driven over mountains and flat land and I just let it do it's thing. I've not noticed it dropping below the torque curve before it shifts but then it has a GM engine in front of it and it's kind of hard to drop below the torque curve on those.
It works well.
There are some places where I will downshift.. Mostly because in the RV parks the speed limit is 5-15 MPH and I very much need to keep it in low.
