When should you downshift a commercial vehicle?

Downshifting should happen before entering a curve or starting down a long hill, not during or after. Downshifting before a curve allows the driver to use engine power to maintain speed through the curve and accelerate smoothly out of it. Downshifting before a hill gives the engine compression braking before momentum builds, reducing reliance on the service brakes and preventing brake fade. Trying to downshift during a steep descent is difficult because the increasing road speed may make the gears match impossible to find, and you could end up coasting in neutral at high speed without engine braking. Downshifting only when stopping wastes engine braking opportunity and increases brake wear. Downshifting at mid-range RPM with no reason wastes fuel.