Because I don't know jack shit.
My car has an overdrive function. I pushed the button the other night just to see what happened and a light came on saying "overdrive off". I thought that was strange, I thought that pushing the button would turn the overdrive on.
So, I opened up the owners manuel and all it said was "overdrive is on whenever you start the car. It only goes off if you turn it off".
My question is, what does overdrive do for me? Is it a good thing? Does it burn more gas to overdrive on? Is it harder on the engine?