"Overdrive," as it applies to cars, refers to the gearing of the transmission. Overdrive is achieved when the transmission’s output shaft (the shaft that delivers power to the wheels) spins faster than the input shaft (the shaft that takes power from the engine).
Regardless of the transmission type, the overdrive gear is always the top gear. In a 5-speed manual transmission, the overdrive gear is fifth gear. In an automatic transmission, overdrive would be the fourth gear in a 4-speed automatic. By definition, their ratios should be below 1:1, typically between 0.7:1 and 0.9:1.
In practical terms, shifting to overdrive allows a car to cruise at the same speed but at a lower rpm, resulting in relaxed and more fuel-efficient driving.
To determine if your automatic transmission’s overdrive is working, let your car cruise at a steady speed, say 80 kph. Next, engage overdrive by switching on the O/D button. (Some automatic-equipped cars have no button but instead have D3 and D4 selections on the shift gate. To engage overdrive, shift it to D4 from D3.)
If your overdrive is working, the engine speed should fall by 500 or more rpm without a reduction in the vehicle’s speed.
Now you may ask, "Why not leave the O/D button on all the time or just leave the lever in D4?" There really is nothing wrong with this practice. In fact, it will not damage the engine or the transmission.
The only reason you would want to disengage overdrive is to avoid the annoying "hunting" between 3rd and 4th gears, especially in city driving when the car is driven at speeds from 60 to 80 kph. In this situation, it is better to leave the car in third gear by locking out overdrive. This way, when a sudden overtaking opportunity presents itself, the gearbox need only downshift one gear instead of two.
Proper gear selection, even in an automatic-equipped car, is the key to extracting maximum efficiency and performance.
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