Why should a driver shift down into a lower gear before a long or steep grade begins, rather than waiting until already partway down it?

Explanation

A driver should shift down into a lower gear before a long or steep grade begins, so the vehicle is already in a gear low enough to control speed mainly through engine braking, since relying on the service brakes alone on a long descent can overheat them and reduce their stopping power when it matters most. Shift patterns and procedures, along with the consequences of improper shifting, are part of the required knowledge CDL applicants must have under 49 CFR 383.111(a)(5)(ii) and (iii), but those paragraphs only name these as required knowledge topics; they do not themselves specify downshifting before a grade as the procedure, which is standard driver-training guidance rather than a codified federal rule. Fuel economy and transmission noise are both unrelated side effects, not the reason for the timing, and there is no federal prohibition on shifting gears while a vehicle is moving.

Source: 49 CFR 383.111(a)(5)(ii) and (iii)