What advantage does downshifting provide while descending a long downhill grade?

Downshifting while descending a long downhill grade is beneficial because it utilizes engine compression to assist in braking. When a driver downshifts, they switch to a lower gear, which allows the engine to engage in a way that increases the engine RPM (revolutions per minute) and provides additional resistance to the vehicle's movement. This creates a braking effect through engine compression, which helps slow down the vehicle without relying solely on the brake system.

This technique is particularly important for heavy vehicles, as excessive use of the brakes on a long descent can lead to brake fade, which diminishes braking effectiveness due to overheating. By using engine compression as a form of braking, the driver can maintain better control of the vehicle and reduce the risk of brake failure, ensuring a safer descent.

The other options do not accurately reflect the primary advantage of downshifting. For instance, increasing speed is contrary to the purpose of downshifting, and while downshifting may help reduce engine wear in certain conditions, that's not its main function during a descent. Lastly, downshifting actually requires more driver focus to ensure that the shift is done smoothly and retains control of the vehicle.