What is Overdrive in the car?
Overdrive is the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute (RPM), leading to better fuel consumption, lower noise, and lower wear. The term is ambiguous.
The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is over-geared, and cannot reach its potential top speed, i.e., the car could travel faster if it were in a lower gear, with the engine turning at higher RPM.
Overdrive as a concept seems pretty awesome – normally displayed as a button or switch, it almost seems like a hidden source of power that can suddenly be unleashed with a press or a flick. Unfortunately, it’s the complete opposite, being used in older cars to produce decent fuel economy and less engine noise once up at cruising speed.
Overdrive as a definition is simply having a faster transmission output shaft speed than the input rotation speed of the input shaft from the engine. This means torque is effectively being multiplied in a positive sense so being ‘overdriven’, creating a more effortless driving experience.
‘Underdrive’ is therefore the opposite, where the engine is spinning faster than the resultant rotation of the output shaft. This is experienced in the lower gears (normally up to third gear) which are used for acceleration before cruising gears are employed.
Overdrive’s functionality is to serve as a high gear mode for a vehicle and can be looked at as an added gear that widens the transmission’s shifting range at its top end. By shifting higher, the powertrain reaches lower gear ratios (the higher the gear, the lower the ratio) and allows the engine to perform at lower RPMs during highway cruising of 50 miles per hour or more.
Whereas a vehicle will normally operate at lower gears with greater output and torque, overdrive inhibits acceleration and maximum power. The vehicle is considered over-geared or overdriven, making it so that top speed and performance are sacrificed for greater fuel economy and a more effortless driving experience.
In many current vehicle models, overdrive is a less prevalent feature due to modern transmissions that offer higher gearing (fifth and above) intended for efficient cruising. If you compare cars, you’ll discover the automatic transmissions in new vehicles typically don’t require a driver to manually activate the feature.
When to Turn Overdrive On?
Overdrive should be utilized at cruising speeds on highways and byways. And the fuel economy benefits of overdrive are best seen over longer distances. Whether it is a lengthy daily commute or a family road trip, overdrive will save money at the fuel pump and deliver a more relaxed ride, overall. When a vehicle gets up to speed after freeway entry, initiating overdrive is recommended.
In older cars, the overdrive on-off button is typically found on or around the shifter. A corresponding overdrive indicator light will illuminate in the gauge cluster, but only when the overdrive is turned off. In newer cars, the overdrive function is governed by a vehicle’s Electronic Control Unit (ECU), and will automatically engage when proper speeds and conditions are met.
Some drivers like to keep overdrive on at all times. These are usually people who do a lot of highway driving and are rarely in situations needing deliberate acceleration or pulling power.
When Should You Turn Overdrive OFF?
The primary time to turn overdrive off is when going up or down a steep hill. Both of these situations merit being in a lower gear. When going down a hill, using a lower gear helps your vehicle engine brake. This lets the engine do some of the work for you without wearing out or overheating your brakes. For this reason, some manufacturers give the “overdrive off” switch a name like “grade assist.”
When going up a steep hill, you typically won’t have to turn overdrive off. Your car should be able to figure out that it’s time to downshift to a lower gear, giving you more torque to get up the incline. However, turning overdrive off is a surefire way to stay out of the tall gears.
Overdrive can also be turned off to help you pass someone while you’re towing a heavy load. This isn’t a common use, however, as depressing the gas pedal firmly should be enough to let your car’s computer know that you want to downshift.
With any manual transmission, you don’t want to leave the car in too high a gear, whether it’s an overdrive gear or not. For example, leaving a six-speed car in 5th gear as you slow down to a stop will lug the engine. This is not good for the engine or transmission. Instead, either downshift or leave the vehicle in neutral as you come to a stop.
Overdrive in Automatic vs. Manual
Both automatic and manual cars can offer overdrive functionality. And in both, overdrive serves as the highest gear (with the lowest gear ratio). With an automatic transmission, overdrive is most often an intuitive function initiated by the ECU when the vehicle reaches the proper speed.
In the case of manual transmissions, overdrive is the highest gear of the transmission but has to be engaged by the driver. In a five-speed transmission, overdrive is the fifth gear; in a six-speed transmission, overdrive is the sixth gear; and so on.
The higher the gear, the more efficient the operation at highway speeds. Naturally, with a manual transmission, the driver plays an active role in engaging overdrive by physically shifting into the highest gear.
