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How Does Automatic Overdrive Work?

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Automatic overdrive systems are a form of transmission system offering significant performance enhancements and can also increase the efficiency of your car.

When people hear the term ‘overdrive’ they may think it’s all about speed, but that’s not the focus of the feature. Overdrive can help a car to go faster, but it’s really focused on allowing the engine to run at a lower RPM while maintaining the vehicle’s current cruising speed.

When To Use Overdrive

Overdrive is an incredibly useful feature, and it’s one you should use the vast majority of the time. The only time you’d likely need to turn overdrive off is if your vehicle is climbing a very steep hill or carrying/towing something incredibly heavy. In those situations, it’s beneficial to be able to rev the engine a bit more. It’s also useful to be able to go into lower gears if you’re going down very steep hills.

Many modern engines have ‘smart’ transmissions, which can detect those situations and will shift gear if it detects it’s necessary. Even those can sometimes fail to pick up when they need to come out of overdrive, and in some cases they might drop in and out of overdrive. Frequent shifting could cause the transmission to overheat and become damaged, so you should remember to turn off overdrive if the situation calls for it, and to pay attention to your speed at all times.

The Development Of Overdrive

When cars were first developed, the maximum power required to propel them at acceptable speeds was minimal – indeed, cars didn’t have a lot of gears. However, as more powerful cars appeared gears became more common and important. In the 1960s, the gap between the desired speed of cars and the amount of power they could generate grew a lot, and multiple gears became common. In 1973, the oil crisis made it clear there was a requirement for a ‘cruising gear’.

The most obvious answer to the power problem would be to add more gears, but that’s not always the correct option. Some cars do opt for that rout, but overdrive is a more sophisticated way of keeping the engine operating at a sensible RPM. Instead of adding a separate cruising gar to the transmission, the two-gear overdrive system is used on many cars. Where there’s a front-wheel drive layout, there’s no drive shaft between the gearbox and the final drive – there’s no real ‘direct drive’ option – marketing literature still uses the term ‘overdrive’, but that’s not really what is happening. Rather, to marketers overdrive refers to any extremely-high ratio used for efficient cruising.

How Overdrive Benefits You

Overdrive units consist of an epicyclic gear, which is either hydraulically or electrically operated and that’s bolted behind the car’s transmission. It can either increase the output speed of the unit or couple the driveshaft to the output shaft. This is where the ‘overdrive’ comes from relative to the input shaft.

Overdrive gearing will reduce the car’s engine speed, which reduces wear and saves fuel. In the US, almost all new consumer vehicles have overdrive included as a fuel saving measure. Overdrive will help to keep most engines in their peak efficiency range most of the time – but it’s important to understand when to use overdrive, because if it’s employed at the wrong speeds it can cut into the engine’s efficiency and eliminate any savings.

Overdrive use should be minimized at lower gears. Car designers often provide high-ratio first gears, and more gears between the first and the last gear, to ensure that the engine stays at its most efficient throughout the full speed range. That’s why it is so rare to see more than one overdrive – except in trucks and high performance cars, where a double overdrive is sometimes included. Even in the performance car market, the second overdrive is only used for very high speeds.

In the end, overdrive is now a common feature and serves to save wear and tear and reduce fuel consumption, which is good for everyone!

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