When should a driver select the gear for descending a steep grade, and which gear is generally correct?

Explanation

The correct gear is selected before starting down the grade, at or below the gear that would be needed to climb that same hill, so the engine can help hold the vehicle back through engine braking rather than leaving the service brakes to do all the work. 49 CFR 383.111(a)(19)(ii) names selecting the right gear as required knowledge but does not specify which gear or when to choose it, so this is standard driver-training content, attributed to the manual. The wrong options fail in two ways: one waits until the vehicle is already moving, which is exactly the timing this practice is meant to avoid, while the other two both remove or minimize engine braking, either through an economy-focused gear choice or coasting in neutral, leaving the brakes to absorb heat they were never meant to handle alone.

Source: 49 CFR 383.111(a)(19)(ii)