On a 2001 Ford Ranger, overdrive is the highest gear in the automatic transmission. It's designed to allow the engine to run at a lower RPM at higher speeds, improving fuel economy and reducing engine wear. When overdrive is engaged (usually indicated by a light on the dash), the transmission shifts into a higher gear ratio than its other gears, resulting in slower engine speed for a given vehicle speed.
If you disengage overdrive (often by pushing a button or moving a lever), the transmission will stay in a lower gear, which provides more power for acceleration, hill climbing, or towing, but at the expense of fuel economy.
