Before the 1980s, the majority of American cars and light trucks came with transmissions that had 1:1 ratio for high gears; this means that the drive shaft of the car will rotate at the same rate in the same direction as an engine. 

The 1:1 ratio has served us well for a period of fifty years or more. As the price of oil increased and the nation as a whole became worried about the amount of air pollution emitted by automobiles, driveshaft professionals began looking for overdrive transmissions as a component to solve the problem.

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This decrease in engine speed is a benefit in many ways:

1.) Lower consumption of fuel – when you travel on the main roads, your vehicle will consume about 1/3 of the fuel.

2.) Lower emissions – when you are on roadways, your vehicle will emit around one-third less pollutants.

3.) A longer engine life – All other things being the same, your engine theoretically has a lifespan that consists of a set amount of rotations. 

5) Reduced cabin noise the engine running at a lower rate will be more quiet, making your trip less stressful. It's easier to hold an exchange of words and even listen to the radio!

In the end the overdrive transmissions have been one of the most significant technological advancements to automobiles over the last 30 years. They have created a bigger improvement in the fuel efficiency of highways than computerized fuel injection and engine controls.