An overdrive unit can be a secondary gearbox or a top or fifth gear addition to a standard gearbox. Its design changes the gear ratio of the vehicle so the engine does not have to work as hard to maintain high speeds. It can be operated manually via a shift lever. The unit has many advantages over the old style transmission types, where gear selection was limited to mostly three forward gears.
An overdrive gear, typically a fifth gear added to a four-speed manual transmission, can reduce the drive ratio by as much as 0.782 to 1. Such vehicles equipped with a fifth gear overdrive option (or standard feature) can gain an increase in speed of 27.88 percent. That translates to speeds of 55 MPH up to 70 MPH. Load and incline play a significant role in minor overall speed adjustments, but for a level and flat driving surface, the speed increase has profound effects.
Since overdrive can change the gear ratio in favor of a much higher gear, the vehicle's engine RPM reduces significantly so it does not have to work as hard. The increase in full economy from overdrive can be as much as 28 percent overall, and this takes into consideration a combination of city and highway driving. For example, it would be like paying $1.50 per gallon at the pump when the price is $1.90. Purchasing 1,000 gallons a year would yield an annual savings of approximately $400. The more gallons purchased, the higher the savings. If most of the driving is on the highway, with very little city driving, the overdrive option shows even higher fuel savings than a combination of city and highway driving. Truck drivers, for instance, use the overdrive or highest gear option for the obvious fuel savings for extended highway driving.
One of the best overall features of an overdrive unit is the significant reduction in wear and tear on the engine and all of its components for the same distances traveled. When the engine turns less, so do the major engine parts, such as the crankshaft, bearings, journals, pump shafts, belts, pulleys and drive train components. Engine parts that rotate or spin less run cooler and for less time. Overdrive units actually extend engine life and can even reduce maintenance and service intervals. Overdrive units also reduce noise factors and afford more comfortable driving conditions.
