Räy Fischer wrote:
That is why saying that "Use this gear for this hill" doesn't work. What
you end up doing usually is trying to pick the gear that gives you a
legal speed descent with the engine in the middle of the power band for
the engine. The trick is to add all the variables up to find that gear...

So, is the calculation involved in adding up all those variables based
on science, or is it an art?

Both. While you could set there and calculate out the gear ratio using
the weight of the vehicle, contact patch friction available, amount of
available engine braking, amount of wind drag, degree of grade, and the
rest and probably come up with it, folks with experience driving can
look at the situation and decide, "Well for this hill 10th should do
it", shift and be down the hill...

Just takes practice and knowing what your rig will do with the given
load. Most of the time a good driver will have an idea about how the
load will handle after about the first mile or so pulling it. Just in
the way it feels through the wheel and the way you shift to get moving.
Every load will act different depending on overall weight, center mass,
center of gravity, wheel spacing, number of drive axles 5th wheel
location, road conditions even the wind resistance and drag under the
trailer all make profound differences.

Now add in people tailgating, cutting you off, erratic speeds, long
hours, and all the rest and it gets a bit stressful in that rockin'
chair. It's one of the reasons there is a shortfall of drivers out there.

--
Steve W.