When driving in mountainous conditions or with a heavy load, turn off the overdrive on the vehicle's transmission. Turn off the overdirve when you are operating the vehicle at a speed that causes the transmission to shift between direct drive and overdrive continually or when engine braking is desired.
Nearly all vehicles manufactured in 2014 have an overdrive gear. It is the highest gear on both automatic and manual transmissions. Under normal operating conditions, operating the vehicle in overdrive provides improved fuel economy. Overdrive uses the gear ratio to produce more speed for the vehicle with fewer revolutions of the engine, but sacrifices power.
Shifting the overdrive unit off increases the power to the wheels. Vehicles need this power with a heavy load, such as when towing a trailer or climbing a steep incline. Using the gear selector or turning off a switch limits the vehicle's access to the highest gear. This limits the fuel economy, but it provides the power to move the load down the road.
When the vehicle is operating down a steep incline, gravity tends to increase its speed. Operating the vehicle with its overdrive off under these conditions allows the use of engine braking to prevent over-acceleration.