Overdrive is an extra gear in a car or truck that lets the engine run more slowly while the vehicle maintains its speed on the highway. When a car shifts into overdrive, the engine doesn't have to work as hard, which saves fuel and reduces wear on engine parts. It's like switching from running to walking while still covering the same distance: you use less energy doing it.

Before overdrive became standard, drivers on long highway trips heard their engines roaring constantly at high speeds. Overdrive changed that by allowing the wheels to turn faster than the engine speed would normally produce. Modern cars shift into overdrive automatically, but older vehicles had a button or switch that drivers controlled themselves.

The word also means a state of intense, high-speed activity. When someone goes into overdrive, they're working at maximum capacity, like a student in overdrive finishing three assignments the night before they're due, or a restaurant kitchen shifting into overdrive during the dinner rush. This meaning captures that sense of pushing beyond normal operating speed to meet a demanding situation.