Overdrive Transmissions - the gears of things. 

in the 60's & 70's most of the vehicles had 3 speed automatics or 3 or 4 speed manual transmissions.  As the end of that era grew near, manual transmissions were available in 5 or 6 speed configurations while automatics now have 6, 8 or 9 speeds. Why? Fuel Economy! Better torque curve! Lower RPM's at Cruising Speeds! Quieter! Most transmissions today offer some form of overdrive.

Overdrive is when the transmission output shaft is turning faster than the engine crankshaft or less than 1:1 ratio. THe allows for less RPM's at the same cruise speed than without overdrive.If you have an older car with only a 3 speed automatic, or a manual shift with 3 or 4 speeds, you might want to consider some kind of overdrive upgrade in the future.

Depending on your application, some upgrades are relatively simple.  Example: a Chevrolet 200R transmission is a 4 speed transmission and is direct replacement for a Turbo 350 trans. Some other conversions require fabrication work.  It is best to look at all available options before heading down path.  Companies like Gear Vendors  make bolt-in over-drive units you can purchase. Usually, this requires a new driveshaft to be made.  The fuel economy you get from the upgrade will offset the cost to do the job.