It wasn’t that long ago that overdrive transmissions were not standard equipment in every new vehicle on the road. The tried and true 4-speed manuals and 3-speed automatics were the only thing we knew, and they worked well, so why would anyone want to change that?
During the late 1970s and early 1980s we saw a push to create more economical cars that produced lower emissions.
As most of us know, it had a lot, if not all, to do with the fuel shortages and the government requiring the automakers to comply with EPA regulations on fuel consumption and emissions.
Thus the mainstream overdrive transmissions and pollution control systems were born into existence at the expense of horsepower. All of those things combined to get the overdrive off on the wrong foot with most car enthusiasts.
What is Overdrive in the car?
Overdrive, simply put, is used as a way of improving fuel economy and reducing engine noise once a car is up to speed. That’s significantly less exciting than IRL hyperdrive.
Back in the day, top gear in most cars meant the input rotation speed from the engine was the same as the output shaft from the transmission or a 1:1 ratio.
That would leave engines sitting at high revs when at a constant speed. Keep in mind this was at a time when anything more than a four-speed gearbox was a rarity.
Rather than developing more expensive gearboxes with more gears built within, it was easier and cheaper to make an additional gearing unit on the back of the gearbox before the prop shaft. And thus, ‘overdrive’ was born.
This, in essence, increases the ratio of the tallest gear. The output shaft has further to rotate, reducing the revs of an engine once at speed.
How Does Overdrive Work?
The input shaft of your automatic transmission is connected to the engine by way of a hydraulic coupling mechanism (the torque converter filled with transmission fluid) that allows the engine to run freely at low RPMs while the transmission is in gear.
That way, you can stop at a light without stalling. As you accelerate, the torque converter transfers power and the transmission responds in first (low) gear. You speed up and the transmission shifts from low gear to higher gears, similar to how you might manually change gears on a bicycle (or manual transmission car).
Each gear is matched to the effort the engine needs to offer for a given speed and amount of resistance from the wind or steep incline. And with each gear, the ratio of the speed of your engine to the speed of the transmission output changes.
Now, every engine has an optimal internal speed (RPMs) at which it runs at maximum power. And every car meets increasing amounts of resistance as it speeds up on the road.
As long as you are accelerating, there is a range of gears (gear ratios) that can automatically match the engine speed to that resistance. For maximum fuel efficiency, the best gear is the one that results in the lowest engine speed for the circumstance.
When you are on the highway driving at a high rate of speed, there is a point at which the speed of your engine operating at peak power equals the amount of resistance to your vehicle.
And there is one specific gear ratio to accommodate that balance. Once you have accelerated to cruising speed (with your transmission in overdrive mode), overdrive is engaged and the input-to-output ratio changes.
Instead of the engine and input shaft of the transmission turning faster than the output shaft (underdrive), or turning at the same speed (direct drive), overdrive causes the output shaft to turn faster than the engine. That way, you can cruise faster with your engine running slower.
Over time, and since the development of front-wheel-drive systems, the definition of “overdrive” has changed. So has the way a transmission achieves the proper gear ratio at high speeds. But in a typical overdrive system, the transmission shifts its way up to cruising speed.
Once acceleration levels off, the torque converter locks into the higher overdrive gear, effectively changing the gear ratio so that your car can travel faster at lower engine RPMs.
You can feel the change as your engine relaxes when you ease off of the throttle to stop accelerating. And you can feel the torque converter unlock when you press on the gas pedal again. Or when you have to climb a steep hill.
To engage overdrive (if your car is so equipped), you either press a button on the side of the shift lever or place the lever in “Drive” (a “D” enclosed in a circle represents overdrive mode).
When you want to deactivate the overdrive, you simply shift to the highest gear available below the overdrive setting or turn it off with the button.
When to Turn Overdrive On?
Overdrive should be utilized at cruising speeds on highways and byways. And the fuel economy benefits of overdrive are best seen over longer distances.
Whether it is a lengthy daily commute or a family road trip, overdrive will save money at the fuel pump and deliver a more relaxed ride, overall. When a vehicle gets up to speed after freeway entry, initiating overdrive is recommended.
In older cars, the overdrive on-off button is typically found on or around the shifter. A corresponding overdrive indicator light will illuminate in the gauge cluster, but only when the overdrive is turned off.
In newer cars, the overdrive function is governed by a vehicle’s Electronic Control Unit (ECU), and will automatically engage when proper speeds and conditions are met.
Some drivers like to keep overdrive on at all times. These are usually people who do a lot of highway driving and are rarely in situations needing deliberate acceleration or pulling power.
When to Turn Overdrive Off?
Overdrive should not be used when you need your vehicle’s full power to accelerate for passing, climb a hill, tread an off-road surface, or trailer a weighted load. All of these applications require a vehicle’s torque to be willfully utilized.
Overdrive restricts that kind of output and may render the vehicle underpowered in certain maneuvers.
Underestimating your vehicle’s power can have dangerous consequences, especially when attempting to overtake a vehicle or climbing a hill while pulling extra weight. In these situations, it is best to keep overdrive off.
When overdrive is turned off, the transmission will remain fixed to lower gears resulting in greater responsiveness to the accelerator. The engine brake will also become more effective.
Overdrive in Automatic vs. Manual
Both automatic and manual cars can offer overdrive functionality. And in both, overdrive serves as the highest gear (with the lowest gear ratio). With an automatic transmission, overdrive is most often an intuitive function initiated by the ECU when the vehicle reaches the proper speed.
In the case of manual transmissions, overdrive is the highest gear of the transmission but has to be engaged by the driver. In a five-speed transmission, overdrive is the fifth gear; in a six-speed transmission, overdrive is the sixth gear; and so on.
The higher the gear, the more efficient the operation at highway speeds. Naturally, with a manual transmission, the driver plays an active role in engaging overdrive by physically shifting into the highest gear.
FAQs.
What is overdrive on a transmission?
Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway.
Should I drive with overdrive on or off?
Leave the overdrive ‘on’ for most driving. If you are not getting up to highway speed, the shift to overdrive will cause minimal wear. At highway speeds keep the overdrive ‘on’ unless you are in the mountains or pulling a heavy load. Lock overdrive out any time you need the engine to control decelleration or traction.
When should I use overdrive mode?
In most cases, it is better to leave the overdrive turned on and let the transmission decide when to shift itself. Overdrive simply means that the output shaft of the transmission is turning faster than the input shaft. At highway speeds, this translates to lower engine RPMs and less fuel consumption for a given speed.
Can I drive my car in overdrive all the time?
Although it might be tempting, you should not use overdrive all the time. There are several instances where it makes sense to use the resource. It’s critical to be careful with this feature to avoid bringing damage to the car when you turn on the mode.
Why would I turn overdrive off?
The O/D off button allows you to prevent an automatic transmission from shifting into those top gears in certain situations, such as when driving through rolling hills, going down steep grades and hauling a heavy load or trailer.