my thumb is on the o/d button whenever I want more power, drive along at any speed and hit the o/dbutton (turn off overdrive, the litte light on the dash will come on) and you will notice the RPM's increase and you will have more power to pass ect... I will cruise on the interstate at around 60 hit the O/D button for an instant and have plenty of power to pass someone.no, it will not hurt the transmission to leave it off, you just won't have Overdrive and you will notice that there should be another shift point at around 45 to 50 MPH. If your the kind of person that doesn't put their foot on the gas all the time, use it when you need that extra oomph going up a hill ect...Overdrive" is the term for gear ratios that are lower than 1:1 – a situation where the engine is turning slower than the drive shaft. Overdrive gears are appropriate when very little torque is needed – the vehicle is no longer accelerating – and can deliver the best fuel economy. The six-speed transmission in the Ford Five Hundred has two overdrive gears, with a final ratio of 0.686:1.i used search and found this:
http://forums.genvibe.com/zerothread?id=15765