Overdrive (O/D) mode is a high gear mode in automatic transmission vehicles that focuses on power, superior speed, and better agility to improve fuel economy.
So what does keeping it in OFF mode mean?
Keeping your O/D in OFF mode stops your car from going into the highest gear. In essence, it locks it to the lower gears (first, second, and third) for better torque response in certain situations, such as when towing heavy loads or going up a steep hill.
Press the O/D button on the gear shifter to turn overdrive OFF since it’s usually ON. By looking at the instrument panel for an O/D OFF sign, you’ll know that your vehicle is no longer in overdrive.
If you want to turn it on again, press the same gear shifter button, and the O/D OFF light on your instrument panel disappears.
But why and when would you want your overdrive OFF? Read on to find out.
The overdrive feature in most automatic vehicles refers to the highest or top gear in a four-gear transmission.
It enables the transmission to turn the driveshaft faster than the engine’s crankshaft.
Generally, the O/D will try to squeeze a few extra miles out of each liter or gallon of fuel by limiting throttle response and pushing the vehicle to higher gears faster.
And depending on the vehicle, some smooth drivers might fail to notice when their car is in overdrive and when it’s not.
More aggressive drivers that brake harder, accelerate faster from a stop, or drive high-performance vehicles will feel when the vehicle is in overdrive.
Automatic transmission vehicles are automatically in an O/D ON mode, and you’ll have to turn it off when necessary.
Hence, drivers who drive on city roads or level highways might never need to turn the O/D feature off.
History of Overdrive
To understand what overdrive is, we’ll have to go through what underdrive and direct drive systems are.
An underdrive transmission, much like the one used in pulley systems, slows the rotation rate in a system by either making the driven pulley larger or the crank (drive) pulley smaller.
In vehicles, an underdrive causes the engine’s crankshaft to rotate faster than the driveshaft- making for faster acceleration.
For example, the first gear is an underdrive one as the engine rotates two or three times faster than the driveshaft.
The second gear falls in between and the crankshaft rotates marginally quicker than the driveshaft.
The engine rotates at the same speed as the drivetrain in a direct drive system. In essence, it’s more of a 1:1 drivetrain to crankshaft gear ratio.
Most vehicles have the third gear as their direct drive.
The direct drive system is beneficial as it experiences no backlash and has a smooth torque transmission.
Also, here are some additional benefits associated with direct drive systems.
- Increased efficiency due to a lack of power loss that’s common with underdrive and overdrive systems.
- It’s simpler to design.
- Can deliver high torques over a wide range of speeds.
The overdrive became a standard feature when it was evident that the automobile market desired efficient cars.
And, fuel efficiency became a point of concern for most vehicle manufacturers, who then responded by adding a higher (overdrive) gear to their transmissions.
The higher gear puts the car in an overdrive mode, whereby the crankshaft rotates slower than the driveshaft.
The effect is that it alleviates the stress placed on the engine, making it burn less fuel.
Additionally, less strain means that your vehicle’s engine runs smoother and experiences less noise and drivetrain wear.
But why should vehicles have an O/D OFF feature? Let’s look at the importance of keeping overdrive off.
Why Use the O/D OFF Button?
As we’ve seen, cars are automatically in overdrive mode (O/D ON) since most drivers prefer improved fuel economies over performance.
The crankshaft must have sufficient mechanical advantage over the driveshaft to get it going when starting a vehicle.
The overdrive kicks in once you’ve achieved a cruising speed (typically above 30 MPH).
However, having overdrive on permanently is pretty challenging when driving on hilly grounds.
The higher gear strains the engine on uphill grades, forcing your vehicle to step down to a lower 3rd gear.
It even gets more complicated when going over rolling hills or a road with many dips and crests.
When driving on such roads, the transmission has to shift down to 3rd gear to enable you to drive uphill smoothly.
It’ll then have to shift back to the O/D gear when going downhill and back to the 3rd gear when going uphill.
As a result, driving with O/D ON on such roads is annoying for the car occupants and overtaxes your vehicle’s drivetrain. It’s better to keep the O/D off when driving on unpredictable grounds.
Actually, there are more times when you’ll need to keep it off.
When to Keep Overdrive OFF
As we’ve seen, it’s better for your drivetrain if you keep your overdrive feature off when driving through rolling hills.
However, some vehicles have sufficient power to cruise through less steep rolling hills even when in overdrive.
Regardless, there are times when you need to keep the O/D off, and here are two situations.
When Towing Heavy Loads or Carrying Cargo
When carrying a cargo close to your vehicle’s payload or towing heavy trailers, keep the O/D OFF.
You’ll need to press the O/D button in most cars to deactivate it since it’s active by default.
Towing or carrying heavy loads forces the vehicle to shift down to lower gears hence sacrificing performance for greater torque.
Keeping your car in overdrive essentially locks it to the fourth low torque gear.
If the O/D remains on while towing, you’ll immediately feel that your car is less responsive and sluggish. It strains your vehicle’s engine and may stall it.
On the contrary, you can keep your O/D ON if you’re towing lighter loads on flat or gently sloping roads.
When Driving Up a Steep Hill
You might want to keep the O/D OFF if you’re driving up a steep hill and feel that your vehicle is struggling.
As we’ve seen, keeping the O/D ON locks your vehicle to the upper gear.
In most transmission systems, the upper gear is always overdriven and performance-based with the least torque.
Vehicles need maximum torque when going uphill or starting; hence always keep the O/D OFF in such situations.
When Driving Down a Steep Hill
Another situation that might necessitate turning the O/D OFF is when you’re driving downhill and would like to use the engine brake.
It’s especially the case when you’re towing a heavy load that may build up momentum and cause your vehicle to accelerate.
The good news is that your vehicle’s electronic control unit (ECU) won’t put the transmission in overdrive unless necessary.
Is it Bad to Drive With O/D OFF?
No. Driving with O/D OFF is crucial when towing or driving up a steep hill if you feel that your vehicle is underpowered.
Driving with O/D ON in such situations damages your vehicle.
Furthermore, at all other times, drive with your O/D ON to give your vehicle a better fuel economy, less noise, and less engine and drivetrain wear.
Here are the advantages of keeping your vehicle’s O/D ON while cruising:
- Better fuel economy
- An extended engine life
- Less noise production due to reduced engine strain
- Fewer emissions as your vehicle’s engine consumes less fuel per mile
- A greater drivetrain accessory life
Final Remarks to Keeping Your O/D OFF
Remember that keeping your O/D OFF is crucial when towing close to capacity or when driving up a steep road, or when it’s necessary to improve your vehicle’s torque performance.
In all other situations, keep it ON until you feel that it’s essential to turn it OFF.
Locking your vehicle to the lower gears lowers its speed while keeping the RPMs high.
The result is that your vehicle will pull harder, become less responsive, and have lower fuel economy figures.