2005 Mazda MPV overdrive

Overdrive is basically another gear that is "higher" (lower numerically) than the rest of the gears in the transmission (and sometimes the OD gear is in a separate drive unit) Overdrive was originally included in cars for cruising on the highway or for increasing overall top speed. The effect of using overdrive is that for a given speed, your car will run at a lower rpm than in a lower gear, saving fuel or it will allow you to have a higher top speed at peak power-producing rpm. With the advent of 5 and 6 speed manual transmissions and now 7 speed automatics, the need for a gear called "overdrive" is obviated since in most modern transmissions the highest gear functions basically like an overdrive.