Sun | Drive
Q: I have a 1998 Mercury Villager with an automatic transmission. The gear shift lever on the steering column has an overdrive on/off button. I've been given contradictory answers by several mechanics as to whether or not I should drive with the O/D activated (the default setting), or if I should push the button off, deactivating the O/D.
Some mechanics have told me that the button should be off in most cases unless I'm driving on a level stretch of freeway. Others have said leave the O/D on unless you are towing a trailer. The owner's manual implies that the only time the O/D should be off is when extra braking is needed, such as descending a steep hill. Does having the O/D on result in more wear and tear on the transmission, particularly in city driving conditions? Does it make sense to deactivate the O/D unless I'm driving at freeway speeds? - John Seward, Roseburg, Oregon
A: The transmission's main purpose in life is to match engine speed to torque demand.
When you pull away from a stop, a lot of torque is needed to get the car rolling, but once the car is up to speed, only a little torque is needed to keep it cruising.
A transmission manages torque by changing gears. In a typical four-speed automatic transmission, first and second are underdrive gears (engine speed is greater than driveshaft speed), third is a direct drive gear (engine speed is equal to driveshaft speed), and fourth is an overdrive gear (engine speed is less than driveshaft speed).
Torque demand varies directly with changes in vehicle speed. The engine applies torque to the drive wheels during acceleration, and the drive wheels apply torque to the engine during deceleration. The transmission sits in the middle, absorbing it all.
An overdrive gear can reduce engine wear and improve fuel economy, but only if it's used under conditions of light torque demand. When a transmission is in overdrive, the torque multiplying relationship between the engine and the drive wheels is reversed, so overdrive combined with high torque demand spells trouble.
Too much torque demand during acceleration can cause the engine to bog or die. Too much torque demand during deceleration can damage the transmission and clutch.
Some overdrives act as an auxiliary transmission unit, and effect the ratio of each gear in the transmission, but the overdrive in your Villager is merely a fourth gear.
Switching the overdrive off on the Villager only effects the transmission's ability to shift into overdrive - it effectively becomes a three-speed transmission, the top gear being direct drive.
If the automatic transmission control system is doing its job, your Villager should rarely have the opportunity (or need) to shift into overdrive during city driving.
There are, however, some transmission shift strategies that skirt the fine edge between transmission durability and fuel economy in the name of satisfying Corporate Average Fuel Economy requirements.
My advice is to use common sense. If you can feel the engine lugging, switch the overdrive off. If you can't, leave it on.
Under no circumstances though, should you ever tow a trailer, or coast down a long grade, with the overdrive switched on.
(Chip Keen is an ASA-certified Master Automotive Technician and the owner of Hansville Repair. Readers may send questions to: Car Forum, The Sun, P.O. Box 259, Bremerton, WA 98337, or e-mail .)
Denny's Driveshafts - Speed Calculator
When building a car or truck for racing or street use this handy Potential Speed Calculator will help you to decide whether to use an overdrive transmission or not and which tire diameter to choose and what rear end gear ratio to pick by telling you what the potential speed of the application will be at a particular engine rpm when you are done. It’s great to be able to select a rear end gear ratio for instance and know that it will perform the way that you intended without making the costly mistake of buying the wrong ratio from the start. It seems that everyone at some time has made a gear change and regretted it because it was too much or too little gear to get the job done as expected. This is just one way this calculator will help you. | Also it will tell you if an overdrive transmission will be better for your particular driving than a non overdrive model in the same vehicle. The tire diameter is also very important and this will also help you to decide which one to choose. As little as one inch in diameter can make a difference to your top speed. It will also let you figure your road speed for a given rpm. It is also a great way to determine driveshaft speed, in relationship to critical speed, and your road speed. This is a great tool to help you make an intelligent decision as to what you need to achieve the desired results. This will help take the guesswork out of building your project. Use it to get some answers and tell your friends about it too. |
FIRST Enter your engine rpm. It can be your top engine rpm or any rpm that you want to figure. Try your top engine rpm first to see what the potential top speed will be then try various other rpm numbers to see what the engine will be running at various highway speeds. It’s very interesting to see what will be going on at what rpm.
SECOND Enter the tire diameter.
The outside diameter of the tire is what you need to enter here. Try selecting various diameters to see what the speed changes will be for various rpm ranges.
THIRD Enter the rear end gear ratio. It’s interesting to see the difference a small change in ratio will make.
FORTH Enter the transmission high gear ratio. For Non overdrive transmissions (Enter
1.0) and many overdrive ratios will be (Enter .7) for instance. Check your model for ratio and enter it in here.
LAST After you enter all the info... Click on the Calculate button to see what the top speed of that particular application would be.
TRY DIFFERENT THINGS Try it by changing the rpm lower and do it once again. Try changing the rear end ratio and do it again. Try changing it from a non overdrive (1) to an overdrive (.7) and see how much the top speed increases. You can spend a lot of time playing with this and you will find the answers are very helpful and if nothing else its entertaining. Enjoy!! And don’t forget to tell your friends about this site.
Special Thanks to CSGNetwork.com for the use of their Potential Speed Calculator Script.