Why Do Some Cars Have An O/D Button?

• 13.09.2026, 20:53

Many drivers remain unaware of its purpose for years.

Owners of cars with automatic transmissions sometimes notice a small button on the gear selector labeled “O/D.” This feature is particularly common in older models. Despite the simple label, many drivers don’t know what this mode does or when they should use it.

Supercar Blondie writes about this.

O/D stands for “overdrive”—a higher gear. Its main purpose is to reduce engine RPM while driving at high speeds. This allows the car to drive on the highway more efficiently and with less strain on the powertrain.

When overdrive is engaged, the transmission allows the drive shaft to rotate faster than the engine’s crankshaft. As a result, at high speeds, the engine runs at lower RPMs, which, among other things, helps reduce fuel consumption.

In most vehicles equipped with this feature, O/D is enabled by default. Therefore, during normal city or highway driving, the driver generally does not need to press this button constantly.

However, there are situations when it’s better to disengage the overdrive. For example, this can be useful when towing a heavy trailer or load, when the transmission needs more torque in lower gears.

Disabling O/D can also be helpful on long downhill stretches. Under certain conditions, this allows the transmission to stay in a lower gear longer and utilize engine braking. This approach helps you better control the vehicle’s speed without constantly pressing the brake pedal.

Thus, the O/D button is not intended for everyday manual gear shifting, but rather for managing the use of overdrive. Under normal conditions, it is best to leave this mode engaged; you should disengage it only when the vehicle requires more traction or additional engine braking.