system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what does overdrive or o/d do in cars?
gpt
Traditionally an automatic gearbox would select the gear based on the output speed so that the engine was turning at the speed at which it provided the maximum power output. This is good for acceleration and hauling heavy cargo. However when the car was light it was not needed and provided lower fuel mileage. An overdrive unit could be mounted after the gearbox and allow you to select an additional manual gear which brought the engine speed down to the optimal fuel consumption range. This saved a lot of fuel on the highway and wear of the engine.

Overdrive units are not so common now as gearboxes have become a lot smarter and is able to select a higher gear itself when it detects that the power is not required. However you might find overdrive being marketed and even used to label a transmission profile suitable for low power highway use.