Over drive changes the gear ratio of your transmission from 1:1 in 3rd to something that allows you to cruise at x speed while reducing the engine RPM required to maintain that speed. It gives you better gas milage and you would normally have it engaged. The transmission will normally shift into and out of OD by itself. The button allows you to override the use of OD and drop into strait 3rd gear for more power when towing something up hill for example.DrewBarney said:I just got 1990 EB Bronco and it has overdrive. what is overdrive I assume it is always on because i press the button and a light lights up and says off? I just wanted to know what overdrive does and is it good to have it on or off?