I’m assuming it’s the overdrive button on the side of my gearshift. 2012 Nissan Altima, base model. Everything I googled assumes I know more than how to change oil. Thanks in advance!!! 😊
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Unless you’re towing or are lugging around bags of concrete in the trunk, you don’t need to hit the button.
OD is overdrive. Overdrive is already enabled, OD disables it.
The “Overdrive” gear is the highest gear in your transmission.
You may have learned about gear or pulley ratios in the past, they work the same as levers.
Engines need transmissions because engines don’t run well at all rotation speeds. Peak fuel efficiency, power output, and torque may all happen at different RPMs and under different conditions.
The transmission lets the wheels turn at a very wide range of speeds while keeping the engine closer to it’s optimal performance range. There are limits to how far a transmission can spread gear ratios before an engine stops being able to perform at its top speed. The higher the gear, the less torque the engine is able to apply to the wheels. Too high of a gear and eventually the engine becomes incapable of sustaining it’s top speeds. Lets say my imaginary car has 5 gears and it can hit the top speed of 200 km/hr in 5th gear. Any faster and the engine doesn’t have the power anymore to overcome the wind resistance, friction, etc. If I add a 6th gear, it does not necessarily increase the top speed of my car. Shifting into 6th gear, my engine can turn slower, but it doesn’t help if it still can’t overcome the wind resistance and friction. In fact, 6th gear can even have a lower top speed because of it, perhaps only 190 km/hr.
In that example, the 6th gear is an “overdrive” gear. It’s literally designed to be oversized. It serves no power or performance purpose. However, at a highway cruising speed of 110 km/hr, it can be more fuel efficient for my car to run in the new 6th gear. The engine gets to turn slower, so it can run at an RPM that’s more efficient and with reduced drag.
Because fuel efficiency is important, pretty much all vehicles have an “overdrive” gear, whether they bother to label it as such or not.
So why do some vehicles have a button to disable it? This is typically found on vehicles with towing options. By disabling overdrive, you’re effectively telling your engine “don’t use the overdrive gear”, just stay in the second highest. This is useful if you’re towing or just where your vehicle is going to be working against more resistance than it would in normal driving. In these scenarios, your engine needs the extra torque output of the lower gear, and your engine will likely be most fuel efficient at the lower load. When towing, it’s pretty easy to be right on the edge of your transmission shifting between 5th and 6th too, and disabling overdrive can make sure that your transmission doesn’t keep trying to change back and forth with unnecessary gear changes while under a heavy load.
Another scenario disabling overdrive can be helpful is during hill descent. When descending a hill, your drivetrain works in reverse. Gravity pulls the car down, wheels turn the transmission, transmission turns the engine. This is all perfectly fine. Fuel injected cars will actually stop injecting fuel too, so the engine just runs off gravity for a while. In a lower gear, your engine spins faster, creating more resistance. So by disabling OD, your engine will shift out of 6th and into 5th and stay there, increasing the resistance. This helps control your speed, and reduces how much work your brakes need to do.
I believe your 2012 Altima has a CVT instead of a traditional automatic transmission. So it doesn’t have discrete gears, but instead two pulleys that can change ratios smoothly and continuously. OD in your vehicle wouldn’t refer to a specific gear, but instead just the top range of your CVT’s pulley ratio.
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