Today, most cars have five or more gears. However, for a long time, cars only had three or four gears, which were required to run at high RPM on the highway. This led to poor fuel efficiency, so car manufacturers introduced overdrive. Overdrive is similar to shifting into a higher gear on a manual transmission or when you’re playing a game – lowering the RPMs.
Overdrive is automatically activated when you start your car. However, you can turn it off by pressing the overdrive (O/D) button or by engaging the gear shifter. So, when would you want to turn off overdrive? There are certain situations where you need to switch off – either when going uphill or downhill.
It is especially useful when going down a steep hill. Since over-rev allows you to shift to a lower gear, it increases RPMs, allowing the engine to slow down on its own without relying on the brakes, giving the driver more control. In particular, burning hard brakes can be dangerous on snowy roads or when towing a heavy trailer. Instead, turning off the car excessively, shifting into a lower gear, and slowing down is a good way to slow down.