When Should I Turn My Overdrive On Or Off?

i think overdrive is always on unless you turn it off. in that case the last gear in your transmission is disabled when you choose to turn off overdrive. lets say a 4 speed automatic transmission with overdrive off will behave like a 3 speed transmission. keep it on i cant think of a case where you would turn off the overdrive unless you are towing a crazy load which you shouldn't do in a car transmission or if you have transmission problems.