Have you ever seen an O/D light pop up on your dashboard and wondered what it’s all about? Well. O/D is an abbreviation for overdrive. This is a feature in many automatic transmission vehicles and is designed to improve fuel consumption when cruising at high speeds.
How Does O/D Work?
Once the transmission detects that you are up to a steady speed and are no longer accelerating hard, it will automatically shift to the highest gear possible.
If you are not going uphill or are not heavy, the engine chooses the overdrive gear that gives it high ratios allowing it to maintain highway speed with the least possible RPM.
At this high gear, hard acceleration will be sluggish and take a split second before the automatic transmission drops a couple of gears to get maximum torque from the engine.
While the gear is in virtually all vehicles, only some give you the O/D light and a button to toggle it on and off. The most common are old model Nissans and Toyotas.
What Does O/D Off Mean
If you see the OD Off light on, chances are you have deactivated the overdrive gear. Switching overdrive off tells the transmission you don’t want the low RPM fuel-efficient performance of the overdrive gear.
This is a common way to activate a ‘sport mode’ that lets your transmission keep the engine within its high power band RPM in vehicles without a dedicated sports mode.
That is why O/D off light is only common in some family Toyota Corollas, Nissan vehicles, and other budget family haulers of the past that didn’t see the need for a dedicated sports mode tuning.
These cars let you disable overdrive by pressing an Off button often located on the shifter and labeled OD.
When you press the button:
- If the car is already in the highest gear, it will immediately drop a gear giving you more torque and grunt
- The O/D Off light will pop up on the dash
- The transmission will no longer shift to higher gear/gears, and your engine will hold higher than usual RPM
- You will notice more engine braking as soon as you are off the throttle
To turn it off, Press the O/D button again. This will re-enable the overdrive gear and turn off the O/D Off light.
When to Use O/D Off Mode
Seeing the O/S off light doesn’t always spell disaster. There are times when you should intentionally activate it for better performance. Some of the instances when O/D comes in handy are:
When Speeding Up to Pass
Since overdrive holds onto lower gears, it will give you more torque from the engine. This will come in handy when accelerating hard.
When overtaking from low to acceptable highway speed with a loaded vehicle or going uphill, you can use it. You can also use it when accelerating to merge.
However, O/D doesn’t give you superpowers. Your engine will have to rev hard to keep the vehicle at speed. This makes it easy to redline and strains your engine for nothing.
- Don’t use overdrive if you are driving slowly. Chances are, the transmission is already at low gear and won’t benefit from limiting the O/D. Most cars slip into OD gear at 50 MPH (this will vary depending on how hard you accelerate, the vehicle type, and load)
- Once the RPMs approach the redline (or are higher than you are comfortable), turn OD on or ease off the accelerator. You don’t want to strain the vehicle more than it can handle
ProTip: If you anticipate you will need to keep off the Overdrive gear, you can still turn it off when accelerating from low speed. The transmission will keep off the overdrive gear once it has cycled through the lest of the gears.
When Driving Downhill for Long
A side effect of holding lower gears for longer is engine lag. The lag forces your wheels to turn slower, reducing your forward speed unless you throttle up and bring engine RPM up.
Engaging OD and keeping your foot off the accelerator could help you tap into this engine braking hence helping you keep your vehicle’s speed in check without constantly riding the brakes.
This can come in very handy when driving down very long hills that could heat up and ruin your brakes.
Manual transmission drivers achieve this by selecting lower gears for better engine braking. This is what those ‘Slow Down and Engage the Right Gear’ road signs mean.
Is it OK to Drive With Overdrive Off?
Driving with Overdrive off is OK and safe – as long as you are driving in the right conditions. There is no point holding your vehicle to lower gears if you:
- Don’t actively need the extra power of lower gears
- Are already traveling too fast for the maximum gear with overdrive disengaged
You can toggle overdrive off for the extra grunt but be sure to turn it back on when you see RPMs going up near the redline, and the transmission isn’t shifting into higher gears.
When Not to Use O/D Off Feature
Using O/D off feature at the right time can give you extra acceleration or better engine braking on long downgrades. However, misusing it will:
- Ruin your transmission
- Wear out your engine faster by keeping it strained at very high RPM for long
- Give you worse gas mileage since high RPM means more fuel consumption.
Some of the situations under which you should avoid O/D off include:
- You are going slow, and the vehicle hasn’t cycled through most of its lower gears.
- You are already driving as fast as your vehicle can go (which is rare) the RPMs are very high. Turning off overdrive and dropping a gear won’t give you extra power. The engine is at its limit.
- When driving up a constant hill. Using it to hold low gears for more grunt up the hill strains your engine for no need. Be content with speeds your engine can handle with O/D activated.
- When you are towing a trailer
How Do You Know if Overdrive is On or Off?
Apart from the hard-to-miss O/D off light on the dashboard, your engine will be at higher than usual RPM when driving with O/D off.
You will get a better throttle response, but the engine braking will also be more aggressive. The car will accelerate faster with your foot on the throttle but reduce speed quickly once off the accelerator, even without tapping on the brakes.
Can Overdrive Mess Up the Engine and Transmission?
Misusing overdrive could mess up your engine and transmission. This will only happen if you keep turning it off under the wrong conditions.
For instance, if you keep it off, holding your vehicle at very high RPM, you could overstrain the engine.
Keeping it off will give you extra torque when towing for long will strain your transmission and torque converter more, making it wear out sooner.
Think of overdrive as you would of gears in a manual transmission. Choosing the right gear at the right time is helpful for your transmission and engine. Turn OD on when you feel your vehicle revving hard and approaching the red zone, and no longer picking up speed.
You have neared performance limits, and keeping O/D off is just wasting fuel and straining your power train for no good reason.
Bottom Line
The O/D off feature was common in older vehicles that did not have as powerful ECUs as modern vehicles.
Modern ECUs use drive modes to change gearbox and throttle response to give you different power delivery and fuel efficiency levels in a way that is better than what you could ever achieve with just toggling O/D on and Off.
That is why O/D Off is no longer a common feature. We now deal with Sport Mode, Eco Mode, Normal mode, and more. With each mode on, you just have to worry about fuel efficiency. The ECU will handle the rest, and you don’t have to actively remember turning it on or off to save your engine and transmission.