When does a car use overdrive?
Overdrive is not for power or accelleration, it allows the engine to run at a lower rpm at any given speed for more economical cruising (faster vehicle speed at lower engine speed, NOT quicker accelleration). For example instead of a 1:1 transmission ratio, overdrive may be 0.67:1 so the engine revs 1/3rd slower (2000 instead of 3000 rpm for example). Typically just high gear would be overdrive, but with a 6-speed, possibly 5th and 6th. For an automatic OD transmission it is automatic. You might use the non-OD position or OD lockout button if towing a trailer, for engine braking going down a long hill, or possibly when scooting around town at low speed if experiencing frequently downshifts. For a manual transmission the proper gear to use should become intuative with experience (enough power without lugging the engine). In the old days before overdrive transmissions were common, it was sometimes a separate device after the transmission or 2-speed rear end, which effectively changes all
