Overdrive Transmission: Function, Working Principle, and Bad Symptoms

Last Updated on May 30, 2019 by themechanic

Hello autocartimes friends, in this post i will explain about basic function, working principle and bad symptoms of automatic transmission overdrive.

Basic Function and Working Principles

The highest gear in an automatic transmission is the overdrive gear. The purpose of the overdrive gear is to reduce the engine’s RPM. This represents the revolutions-per-minute of the engine while it is running. There are always a certain number of revolutions-per-minute taking place in the engine, whether the vehicle is idle or moving.

Of course, when you are driving, you put more demand on the engine which causes the RPM to increase. Normally the engine noises increase as the RPM increases. But if you’re driving at one constant speed on the interstate or some other major road, you can switch to the overdrive gear and reduce the RPM as well as the noises.

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The main benefit of an overdrive transmission function is fuel efficiency. Since the overdrive gear reduces the engine power by lowering the RPM, it also means the engine doesn’t demand as much fuel either. Because of this, your engine becomes more fuel efficient as you drive. If you’re someone who normally uses cruise-control as you drive, then this will benefit you the most.

3 Bad Symptoms

The overdrive function of the transmission will usually stay functional. However, there are times when the overdrive function may wear down or malfunction for whatever reason. There are some symptoms that you will recognize when this happens. Once you recognize them, it is important that you get your overdrive gear fixed or replaced in the transmission. Otherwise, you could end up with either transmission damage or something much worse.

Below are the top 3 symptoms of a bad overdrive transmission.

1) Bad Fuel Efficiency

Since the overdrive transmission function is supposed to increase fuel efficiency, a bad overdrive will reduce fuel efficiency and lower your fuel economy. This means having to spend more money on gas than you’d like. You may not link this problem to a bad overdrive gear right away. But as you experience more of the other symptoms listed here, you will start to figure out the truth.

2) Bad Engine Efficiency

The overdrive gear is supposed to let you drive fast while reducing the stress that is put on the engine. In other words, it helps your RPM stay low while you accelerate your vehicle with the gas pedal. But this kind of engine efficiency can be compromised if your overdrive gear starts to go bad. Then you may notice your RPM increasing as you accelerate rather than decreasing. This will coincide with the problem of bad fuel efficiency too.

3) Vibrations

Another function of the overdrive gear is to allow your driving experience to be smooth and simple. However, a bad overdrive gear could cause you to experience vibrations in the cabin. In actuality, these vibrations come from the drive shaft and can be felt in every seat of the vehicle. As you accelerate, the vibrations will get worse.


The overdrive gear is actually available for both automatic and manual transmission vehicles. The only thing is that most vehicle manufacturers do not make their vehicles with an overdrive gear. This ends up being something that you need to have installed separately as an add-on. You basically need to purchase an overdrive kit, which can cost you anywhere between $1,000 and $3,000 on average. On top of that, you’ll need to hire a mechanic to install the overdrive gear for you. Then you’re looking at labor costs on top of the cost of the kit.

You should only consider purchasing this kit if you think you can really benefit from the extra savings on fuel and energy efficiency. The engine, in particular, may have a longer lifespan if you decide to go this route. It all depends on how long you want to keep your car.

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