Overdrive in North America
In the days before automatic transmissions were common, especially in the 1950s, many rear-wheel drive American cars were available with an overdrive option. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedal, the action was much like a semi-automatic. Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic. A knob connected to a bowden cable, similar to some emergency brake applications, was also provided to lock out the unit mechanically. Using overdrive with the main 3-speed transmission in 2nd gear was similar in ratio to 3rd gear, and with the main transmission in 3rd, the overall ratio was fractional (i.e., "true overdrive").
Such add-on overdrive boxes were available from the 1930s to the 1970s for cars and light trucks.
Today, most gas and diesel cars and trucks come with an overdrive transmission because of the benefit to fuel economy. Overdrive is included in both automatic and manual transmissions as an extra gear (or two in some cases).
Since 1979, add-on overdrives are provided by Gear Vendors, Inc., El Cajon CA. for most U.S. made vehicles from the 1920s through to current (2010) cars and light trucks.
Read more about this topic: Overdrive (mechanics)
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