* Towing or hauling heavy loads: Overdrive reduces engine RPM, which is less efficient when under heavy load. Turning it off will keep the engine in a lower gear for more power.
* Climbing a steep hill: Similar to towing, you might need the extra power and lower gear ratio that comes from disabling overdrive.
* Experiencing significant engine braking: While generally not necessary, some drivers prefer the extra engine braking on steep downgrades without overdrive.
In most normal driving situations (city driving, highway driving at steady speeds), overdrive should be ON. It increases fuel efficiency and reduces engine wear by allowing the engine to run at lower RPMs at higher speeds. Leaving it off for extended periods at highway speeds will increase engine wear and decrease fuel economy.
