* Highway driving: Overdrive (usually indicated by "OD" on the shifter or dash) should generally be ON for highway driving. It allows the engine to run at lower RPMs at higher speeds, improving fuel economy and reducing engine wear.
* Towing or hauling heavy loads: Overdrive should usually be OFF when towing or hauling heavy loads. This prevents the transmission from hunting (shifting up and down repeatedly) and keeps the engine in a lower gear for better power and control, especially on inclines.
* Mountainous terrain or steep inclines: Similar to towing, overdrive should likely be OFF to maintain sufficient engine power for climbing hills.
* City driving or stop-and-go traffic: Overdrive can be left ON or OFF depending on your preference. Leaving it on might improve fuel economy slightly, but turning it off can provide slightly better acceleration from a stop. It's largely a matter of driver preference in this situation.
In short, there's no single right answer. Use your best judgment based on the driving conditions. Pay attention to how your truck performs; if it struggles or the transmission hunts excessively, turn overdrive off. If it feels smooth and efficient, leave it on.
