What is the primary reason for using lower gears when descending a hill?

Using lower gears when descending a hill is primarily about controlling vehicle speed and minimizing brake usage. When a truck is in a lower gear, the engine can effectively use engine braking, which slows the vehicle down as it travels downhill. This method of controlling speed reduces the reliance on the brake system, thereby preventing overheating and potential brake failure that can occur from excessive braking.

The lower gear allows the engine to maintain a higher RPM, which generates sufficient resistance to slow the vehicle without constant brake application. This practice is crucial for maintaining safe control of the vehicle while navigating steep declines.