Unless you are in snow, I wouldnt select the ECT positon. Turning overdrive off however does not hurt the car, just increases engine speed and stops the dreaded searching for overdrive in city slow traffic. I have actually instructed my wife to turn the overdrive off while going through the city at stop and go traffic. Mileage goes down a little, maybe 1mpg but big deal, I feel the tranny is not doing such a workout trying to save fuel while going slow... by the way I used the ECT button while up at Tahoe, and left it on and was going faster than 50MPH and when I stop there was a definite burning smell (not good) Read the book and they do not recommend above 50 while in this position. It may be slower than 50 but I remember whatever it was I was going to fast. I think it is in the owners manual....