Prior to the 1980s, most domestic cars and light trucks had transmissions with a 1:1 ratio in high gear, which means that the car’s driveshaft will turn the same speed as the engine. This 1:1 ratio served us well for fifty years or more. As oil prices rose and the country as a whole became increasingly concerned about the amount of air pollution coming from our vehicles, automakers began to look towards overdrive transmissions as part of the solution.
With an overdrive transmission, the top gear is less than a 1:1 ratio, which means that the driveshaft will turn at a faster speed than the engine. For example, if you have a car without overdrive and a 1:1 top gear ratio, a 3.08 axle ratio, and a 26″ tall tire, your engine speed at 70 MPH is about 2750 RPM. The typical overdrive in a domestic car is about a 0.70:1 ratio, which means that in top gear the driveshaft will turn 42.9% faster than engine speed (1 divided by 0.70 = 1.429). In the same car with an overdrive transmission and a 0.70:1 top gear ratio, engine speed at 70 MPH is reduced to 1925 RPM! That is 825 fewer RPM, a reduction of almost one-third.
