Domestic vehicles and light trucks had 1:1 ratio transmissions in high gear before the 1980s. This means that the car’s driveshaft will rotate at the same speed as its engine. For fifty years, this 1:1 ratio was a reliable choice. Overdrive transmissions became more popular as oil prices rose and people became progressively concerned about air pollution from cars.
An overdrive transmission will have a lower top gear than a 1:1 ratio. This means that the racing driveshaft turns at a faster rate than the engine. If you drive a car with an overdrive transmission and have a 1.08 top gear ratio, 3.08 axle ratio, and a 26-inch tall tire, your engine speed at 70 mph is approximately 2750 RPM. You can also check out here to get more information about the racing driveshaft.
Image Source: Google
A typical domestic overdrive ratio is 0.70:1. This means that the driveshaft in top gear will turn 42.9% faster than the engine speed (1 divided with 0.70 = 1.292). The engine speed of a car equipped with an overdrive transmission is reduced to just 25 MPH when using a top gear ratio of 0.70:1. This is a decrease of 825 RPM, almost one-third.
This shrink’s engine speed offers several benefits:
- Lower fuel consumption – On the highway your engine will consume roughly one-third less fuel.
- Lower emissions – On the highway your engine will produce nearly one-third less pollution.
However, there are some minor negotiations. When the transmission is in high gear, the engine will not have as much power to pass and go up hills. Downshifting may be essential at times. Although overdrive transmissions tend to be slightly heavier than non-overdrive complements in some cases, this change is slight.