Overdrive is simply a set of gears designed to provide fuel efficiency at high speeds. Unlike low gears, overdrive has a gear ratio of less than 1.0:1. This means that the wheels of the vehicle rotate more with each engine rotation, allowing the engine RPM to remain low while the speed remains high.
The opposite of an overdrive gear is a granny gear, which provides lots of torque to the wheels, but only at very low speeds.
Which Vehicles Have Overdrive?
Almost all vehicles have overdrive. Both automatic and manual transmission vehicles use overdrive gears.
Debuting in 1934, overdrive was originally a separate system used in three-speed transmissions. Drivers could manually engage overdrive while in 2nd or 3rd gear (at cruising speed). In some cars, letting off the gas pedal would automatically switch to an overdrive gear. Pressing the accelerator again would disengage it. This lasted up until around the 1980s.
As vehicle speeds increased over the decades, so did the need for a way to efficiently, safely, and comfortably travel on the highway. Today, overdrive gears are simply part of the transmission and are engaged naturally during regular shifting.
Modern transmissions usually have two or more overdrive gears at the top of the gear range. Turning the overdrive off prevents the engine from using these gears, which is useful for reasons we’ll discuss below. Most vehicles will have a button (frequently on the side of the shifter itself) to control overdrive. The button usually has the letters “OD” on it or an icon of a vehicle ascending or descending a hill.
Example of Modern Transmission with Overdrive Gears
How Is Overdrive Different In Automatic and Manual Transmissions?
Modern automatic transmissions will naturally shift into their overdrive gears as needed. In a manual transmission car, the driver will naturally shift into the overdrive gears once at high speeds. Unlike older cars where overdrive was engaged with a lever, these gears are used in normal driving without any additional action required.
When Should You Use Overdrive?
In an automatic vehicle, overdrive is always on by default. For most driving situations, you should leave it on. This allows your vehicle to use all of its available gears including the highest ones. Overdrive maximizes fuel economy and keeps NVH (noise, vibration, harshness) at a more tolerable level. There are no downsides to having it on.
In a manual transmission vehicle, overdrive gears are used naturally as the driver shifts to higher gears at higher speeds.
When Should You Turn Overdrive Off?
The primary time to turn overdrive off is when going up or down a steep hill. Both of these situations merit being in a lower gear. When going down a hill, using a lower gear helps your vehicle engine brake. This lets the engine do some of the work for you without wearing out or overheating your brakes. For this reason, some manufacturers give the “overdrive off” switch a name like “grade assist.”
When going up a steep hill, you typically won’t have to turn overdrive off. Your car should be able to figure out that it’s time to downshift to a lower gear, giving you more torque to get up the incline. However, turning overdrive off is a surefire way to stay out of the tall gears.
Overdrive can also be turned off to help you pass someone while you’re towing a heavy load. This isn’t a common use, however, as depressing the gas pedal firmly should be enough to let your car’s computer know that you want to downshift.
With any manual transmission, you don’t want to leave the car in too high a gear, whether it’s an overdrive gear or not. For example, leaving a six-speed car in 5th gear as you slow down to a stop will lug the engine. This is not good for the engine or transmission. Instead, either downshift or leave the vehicle in neutral as you come to a stop.
How Do You Turn Overdrive Off?
Some modern automatic cars have an overdrive on/off button on the shifter. However, it won’t say “overdrive”, usually showing a vehicle on a hill. Others no longer have an overdrive button. Instead, you can manually select a lower gear to get the same effect. An automatic with manual shifting (including those with paddle shifters) will let you shift down to a lower gear of your choice. Some automatics have a gear selection for D4 or D3, which sets the highest gear the car will shift to. Either way, that will keep you out of overdrive.
Is It Bad to Drive With Overdrive Off?
It’s not bad to drive with overdrive off and does no harm to the transmission. However, you will get worse fuel economy and have more noise at high speeds. There’s really no reason to leave it off unless you need to ascend or descend a steep hill. You can turn overdrive on or off at any time or speed.
Sources: Automotive History: Planetary Overdrive, Curbside Classic | The Overdrive Option-Dispelling the Rumors, Myths, MotorTrend
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