system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is Overdrive? Is it safe to take a vehicle OUT of overdrive to travel up an uphill road?
gpt
Overdrive is any gear ratio higher than 1:1.  There isn't anything special about it aside from this.  Some sports cars even have multiple overdrive gears.  For instance, in the Dodge Viper's 6 speed transmission, both 5th and 6th gear are overdrive gears.

In first gear, your gear ratio might be 4:1, for instance.  The engine spins 4 times, but the driveshaft only spins once.  You get 1/4th as much speed but 4 times as much torque.  Great for starting out, or if you have to move something heavy.

Overdrive is exactly the opposite.  Overdrive might be as much as 1:2.  This means the engine spins once and the driveshaft spins twice.  Speed is doubled and torque is halved.

Why does overdrive exist?  For fuel efficiency purposes, you want your engine running as slowly as possible at as high a throttle setting as possible.  Much of the friction inside an engine is skin friction, which increases with the square of the velocity.  This means that running your engine twice as fast gives you something like 4 times as much friction, which means you have to burn more fuel to generate the same amount of power.  Also, you have pumping losses - running your engine at a low throttle setting creates a vacuum in the intake manifold which the engine must work to overcome.  Again, more work being done inside the engine means more fuel spent to get the same amount of power at the driveshaft.