Most automobiles, especially newer models, utilize overdrive transmissions that are push-button operated. These push-button mechanisms are normally located on or near the transmission gear shift mechanisms, which on most newer automobiles are located centrally between the two front car seats. Many older model cars equipped with overdrive transmissions have their overdrive activation mechanisms located directly on the transmission gear shift dial, normally as an extra gear option along with the normal drive, park, reverse and lower gear options.To select overdrive in these older model cars, the transmission gear shift lever is simply dropped into the "O" slot on the transmission gear shift dial.
When the overdrive activation button is engaged on a car equipped with an overdrive transmission, an electrical signal is sent directly to an electrical switch located in the transmission that engages the overdrive transmission gear. When the overdrive gear is engaged, the engine revolutions per minute (RPM) are reduced, which translates into a slower-revving engine, which ultimately produces increased gas mileage. The overdrive gear is a separate transmission gear. Although the overdrive gear can be engaged at any engine speed, it is intended for use at highway speeds, speeds that typically burn more gas and would benefit from the reduced engine work rate provided by the transmission overdrive gear.
The intended purpose of an overdrive transmission gear is to save gas. On cars not equipped with an overdrive transmission, gas mileage is reduced due to the lack of an extra, lower-revving transmission gear. If a car has a 4-speed transmission with no overdrive, highway speeds will be driven in the fourth gear. On a car with a 4-speed transmission with overdrive, the car can be driven in gear number 4 and then switched into overdrive, which is essentially an extra gear, basically a 5th gear, a gear that allows the engine to turn slower and thus save extra gas.
