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http://en.wikipedia.org/wiki/Automatic_transmission
Drive (D)– This allows the car to move forward and accelerate through its range of gears. The number of gears a transmission has depends on the model, but they can commonly range from 3 (predominant before the 1990's), 4, 5 (losing popularity to 6-speed autos, though still favored by Chrysler and Honda/Acura), 6 (found in VW/Audi Direct Shift Gearbox, Toyota Camry V-6 models, the Chevrolet Malibu LTZ, Corvette, GM trucks and Pontiac G8), 7 (found in Mercedes 7G gearbox, BMW M5 and VW/Audi Direct Shift Gearbox) and 8 in the newer models of Lexus cars. Some cars when put into D will automatically lock the doors or turn on the Daytime Running Lamps.
OverDrive ([D], OD, or a boxed D) - This mode is used in some transmissions to allow early Computer Controlled Transmissions to engage the Automatic Overdrive. In these transmissions, Drive (D) locks the Automatic Overdrive off, but is identical otherwise. OD (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35-45 mph (approx. 72 km/h). Under hard acceleration or below 35-45 mph, the transmission will automatically downshift. Vehicles with this option should be driven in this mode unless circumstances require a lower gear.
