Transmission Overdrive Problems

Overdrive is an actual gear within manual and automatic transmission systems. In manual systems, overdrive is usually the fifth gear, while automatic systems show a "D" enclosed in a circle as the extra gear. Overdrive is designed to re-route engine power during periods of increased speed, such as highway driving, according to Samarins.com, a resource site for car buyers and owners. The overall effect enables a car to travel faster while using lower rpm engine power. This redistribution of power and speed is designed to lessen engine wear and tear, while increasing overall fuel economy.

Transmission overdrive gears are particularly useful for highway driving, where increased speed is needed. Normal transmission operation uses the engine drive train, a torque converter and a planetary gear set to transfer engine power to the wheels, or drive shafts, according to Family Car Parts. Overdrive operation uses an epicyclic gear train that's attached to the transmission. This gear train can either re-route engine power, or it can directly increase drive shaft speed on its own. The overall effect works to maximize engine efficiency by directing output power directly to the wheels of the car.

Transmission overdrive causes the mechanisms that work the drive shafts to move faster. The effects of redistributing power can potentially be too much for the drive-shaft components to handle, according to Family Car Parts. The shaft itself is usually a hollow metal tube that has little to no internal bracing. When subjected to overdrive on a frequent basis, the balances that keep the drive shaft from vibrating may begin to give. As a result, the overall ride of the car may begin to vibrate from the increased power assigned to the drive shafts.

The overdrive transmission system is designed to increase engine efficiency, as long as it's used according to the manufacturer's recommendations. As different cars run off of various engine makes and designs, there may be recommended speed ranges where this extra gear will work best, according to Family Car Parts. Optimal engine efficiency should result in fuel savings; however, using overdrive outside of the recommended speed ranges can cause an overall decrease in fuel use efficiency. Another component that may or may not affect efficiency is tire size. Tires that are too small or too large can significantly reduce fuel and engine efficiency when using overdrive or when using the regular "drive" gear.

In some makes and models, the drive shaft components may contain differential gears designed to work in sync with the gears in the planetary gear set. The effects of overdrive usage on a frequent basis can create excess friction within the differential gear set, according to Family Car Parts. These components are bathed in heavy oil to promote minimal friction; however, there is no cooling system in place to prevent overheating. As a result, wear-and-tear problems can develop from the heat and friction generated inside the drive shaft components.