Originally Posted by Morphias87
Short and simple,what exactly is overdrive?
Basically, in the olden days, top gear in a transmission was direct drive which is a 1:1 ratio. Early overdrives were a seperate bolt-on unit using a planetary gearset that when engaged raised the ratio beyond 1:1, that of course lowered engine speed for any given road speed. Overdrive units of this type can still be bought new from a company called Gear Vendors.
Modern overdrive transmissions/transaxles accomplish the same thing in a different way. Most of them are 5 or 6 speeds where 4th gear is the direct drive. The higher gears have overdrive ratios built right in. This saves a certain amount of weight and complexity but accomplishes the same thing.