imbus,

Think of overdrive as both a 4th gear, and a replacement for 3rd. It provides a wider, smoother powerband when cruising at high speed, where third creates a narrow, but stronger powerband when you need to punch the gas and pass a car on the highway. The transmission will shift between 3rd and O/D based on your driving style; if you're doing alot of passing it stays in 3rd, if you're cruising at a constant it stays in O/D. For normal highway driving turning O/D off will kill your gas mileage (because the engine is working harder to keep the car at that speed), but if you're racing it then turning it off is the way to go.

hth,

mark h