available in automatic transmission
The principle that speed and torque can be traded is used in the case of the overdrive gear to increase the speed coming from the engine flywheel at a resulting loss in torque. This is usually a Good Thing when travelling at highway speeds (65MPH or 100KPH and faster, depending on how you reckon speed), and especially if your car is a four-speed.
If your car has a four-speed transmission, it probably starts wasting energy somewhere around 70MPH. It does this by revving higher and therefore producing more power than is necessary to maintain speed, or to provide regular amounts of acceleration in that speed range. What your transmission wants to do is to select a higher gear, so that it can provide an appropriate amount of power for the speed at which the wheels are turning. In a five-speed or six-speed transmission, this is not a problem. Your four-speed, on the other hand, needs another option.
Some cars have a manual cruise control - you press a button, either on the shifter or on the dashboard, and cruise control is turned on. In other cars, the ECM determines whether or not it is appropriate to engage the overdrive. In either case, when OD is engaged, something along these lines occurs over the course of about a second:
- Whatever gear setting the car is currently in (probably fourth) is disengaged.
- The torque convertor is stalled, which dramatically drops the amount of energy transmitted to the automatic gearbox.
- The automatic gearbox engages a gear with a high ratio of output to input. This has the effect of increasing speed at the expense of torque.
- The torque convertor's lockup clutch engages, which has the effect of bypassing the torque convertor completely, thereby transmitting engine power directly to the automatic gearbox. This is very important because of the torque drop across the gearbox when OD is engaged.
When OD is engaged, you will notice that your RPMs have dropped, but your speed will remain steady. Because the engine is turning more slowly, it will consume fuel at a lower rate. Also, you won't be lugging the engine by having it generate unused energy, and that will cut down on carbon deposits and help your spark plugs last longer.
You may be wondering, "Is there really a difference between an OD gear and a regular gear?" Well, yes there is, and you will see why the moment you try to accelerate suddenly while in OD. You will notice the transmission downshifting. This is because acceleration requires torque, which OD has traded off in order to gain speed. Let's say you have (for example) a five-speed car, and your automatic transmission is somewhere between the beginning and the middle of fifth gear, and you accelerate moderately. The car will stay in fifth gear because that gear has the ability to transfer enough torque to increase speed. Tap a little harder on the gas, and it will downshift to fourth, in order to take advantage of higher RPMs and consequentially higher torque. If you had a four-speed with OD, and you were in OD mode, and you accelerated moderately, the transmission would immediately downshift, since the OD gear can't pass enough torque to effect said acceleration.
For a much more in-depth discussion on overdrive, and on automatic transmissions in general, see http://www.howstuffworks.com/automatic-transmission2.htm.