Consumers often mistake “overdrive” to mean that the vehicle is faster, as in “kick it into overdrive!” While it can afford a higher top speed, the overdrive gear is really about efficiency. Generally speaking, the faster an engine runs, the more fuel it burns. An overdrive gear lets the engine run at a lower rpm to maintain the vehicle’s cruising speed.
Turning Overdrive Off
What’s with those overdrive-off switches? Often marked “O/D,” they allow you to lock out the 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. (Each position below Drive successively locks out the gears above the number shown.)
When would you turn overdrive off? Never, or almost never, depending on how you use your vehicle. Under the strain of hauling, towing or climbing a hill, the vehicle’s engine may strain in the overdrive gear. Turning overdrive off downshifts a gear, which allows the engine to rev higher and give you the power to motor on. Likewise, if you’re going down a steep hill, you may want to turn overdrive off — or shift into a lower gear — to allow engine braking to keep the car from speeding downhill. (Standing on the brakes can overheat them and induce brake fade.) Many of today’s transmissions are “smart” enough to downshift on their own when you’re hauling or climbing a hill. But if you find that the car doesn’t downshift or the transmission is frequently going into and out of overdrive, it may be wise to turn it off because frequent shifting can overheat and damage a transmission. Conversely, very few transmissions are smart enough to downshift when going downhill, so here the job is up to you if you find you’re staying on the brake to remain in control.
What’s the Cost?
Driving in a lower gear will almost always burn more fuel. This is why I’m not fond of O/D off buttons mounted on the shifter where they can be tripped by accident. It’s far too easy for an unwitting driver to put on many miles not knowing overdrive is off, which results in poorer fuel economy.
Technically, an overdrive gear should have a ratio lower than 1-to-1, which means the engine turns less than a full revolution for each revolution of the transmission’s output shaft. Because engines must run at a minimum speed of around 500 rpm, and because their output is greater at higher revolutions, the transmission’s job is to gear down the engine’s driveshaft so you can accelerate from a stop. For example, when the car is in first and second gear, the engine’s driveshaft rotates several times for each time the transmission’s output shaft rotates once. Technically, in a transmission lacking overdrive, the highest gear represents a direct connection between the engine’s driveshaft and the transmission’s output shaft — a 1-to-1 ratio. Again, an overdrive gear’s ratio technically is lower than that. In practice, however, some overdrive gears have ratios higher than 1-to-1.
Information for this was taken from Cars.com’s glossary, written by Joe Wiesenfelder.
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