[wolfox]

OD all the time, everywhere - for best gas mileage. I only take it out of OD when hauling loads of cargo, or climbing steep terrain at highway speeds. (There are a ton of hills out here in the Ozarks) Under light to moderate acceleration, proper OD shift will occur at 45 MPH. Heavy throttle like when merging into traffic at highway speeds, it will be delayed until 55 to 65 MPh depending on the circumstances. It will kick down out of overdrive into "passing gear" when pressed down more than 3/4 throttle~floor when moving at most highway speeds.

It's not a power gear, no - but rather it allows the engine to run at much lower revolutions to save gas, wear and tear on the truck. Driving in OD also helps the tranny keep itself running cooler. Google up info on the GM 4L60E overdrive tranny to get all the info on it.