On a long downgrade, what gear should you select?

The traditional rule of thumb for older trucks is to descend in a gear lower than the gear used to climb the same grade. Modern trucks with more powerful engines and engine brakes may allow descending in a slightly higher gear, but the principle is the same: select a gear that lets engine braking and any auxiliary brake (Jake brake, retarder) do most of the work holding speed, so service brakes are used only for occasional snub braking. Never descend in neutral — it is illegal in most states, eliminates engine braking entirely, and means a complete dependence on service brakes that will overheat. Always select your gear before starting the descent; trying to downshift halfway down is difficult and dangerous because road speed may exceed what gears can match.