What does overdrive on a Toyota Sienna do?

I have a 2000 toyota sienna and have a button for overdrive and i have no idea what it's for.What does overdrive on a Toyota Sienna do?Overdirve is a 4th gear for the transmission. It lowers the RPM's at highway speeds to improve gas mileage. Most newer cars do not have a button to switch in and out of OD, its fully automatic. Personally, I like to easily switch out of OD for passing or towing, but its not as common anymore.What does overdrive on a Toyota Sienna do?The overdrive allows your engine to turn slower when running at highway speeds. It greatly improves your gas mileage over not using it. When you turn it on it generally won't engage until you reach 45 MPH or so.