Hey man heres all your info. Refers to the transmission’s highest gear; allows for efficient cruising. The o/d locks out your top gear which means the transmission will not upshift to that gear. Almost any automatic transmission allows you to do the same thing by shifting from Drive (D) to the next lower gear number, typically 4 or 3. Turning overdrive off downshifts a gear, which allows the engine to rev higher and give you the power to motor on. Driving in a lower gear will almost always burn more fuel.