Overdrive in an auto is like 5th in a manual. you dont get much performance out of it, but i helps keep the rpms down while cruising. you will want it on while cruising to help save gas. the only time you would want it off is when going up and down hills continuously, like driving in the mountains or something, to keep the tranny from shifting in and out, causing excess wear. there really is no reason to turn it off except for times mentioned above. you should be able to feel the tranny shifting in and out. if it starts doing this because of the area you are driving in, turn it off. otherwise, leave it alone to save gas. Hope this helps. I tried to explain as best I could.
