Most ppl will have that perception but actually OD means you will have an additional top gear so that the output rev is faster than the INput rev. but this produce low torque for comfort driving in high constant speed at a lower engine RPM.
Imagine a manual car with OD off is only up to 4th gear and when OD is switch to ON, you will get 5th gear.
Simple as that.
edit: typo INput

This post has been edited by spacepilot: May 8 2007, 09:19 PM

This is correct. In other words, Output revs is faster than Input revs.
Take a very simple example, riding a Racer bicycle, have the front gear adjust to the biggest and the rear gear to the smallest, here you have an OVERDRIVE.

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Added on May 9, 2007, 10:12 amThis is the wrong perception most ppl knows, no idea who start it.

This post has been edited by spacepilot: May 9 2007, 10:12 AM