Over-drive, what is it for?

Definition: Any gearset in which the output shaft turns faster than the input shaft. Overdrive gears are used in most modern transmissions because they reduce engine rpm and improve fuel economy. Occasionally, a separate gearbox with an overdrive gearset is coupled to a conventional transmission.

In short, it's a fifth gear for your 4 gear automatic transmission.

Others will refute this but your owner's manual will suggest you leave it on.

On the 99 Camry 4 cylinder the default is ON.

When you press the button (engage it) you actually turn overdrive off.

In heavy traffic and rain and snow I always leave the overdrive off for seemingly better engine performance.

Overdrive only works at higher (not necessarily fast) speeds (4th gear speeds). It will not engage during acceleration but rather at a constant cruising speed. Test it you'll see... On the highway at 100km/h, with o/d on, set the cruise control. Once set, turn off overdrive. Your car will down shift (o/d to 4th), engine will turn faster (rpm) but you wont go any faster. All it does in the end is save gas. 87-91 camrys (I don't know about this feature in later models) have a power/normal(economy) button as well as the O/D button. When set to power, the transmission shifts at a higher rpm thus giving the car better acceleration (faster). High rpm = high gas consumption

"Overdrive" is simply your highest gear (normally the forth gear in an automatic transmission: Low, Second, Drive, Overdrive). Your car will automatically shift into overdrive when your speed and acceleration is appropriate (cruising at moderate to high speeds). When more power is demanded, such as for passing, your transmission will automatically shift to a lower gear. You should normally leave overdrive on, except when you want to prevent your car from shifting to the highest gear (maintaining speed while climbing a steep hill is the most likely occasion). Your car will automatically shift into overdrive when your speed and acceleration is appropriate (cruising at moderate to high speeds).

Overdrive "ON" is the normal position, if you push the button to turn overdrive "OFF", you will see a red or yellow light on the gear indicator that says something like "O/D OFF" to remind you to turn it back on.

If you drive with O/D off, you will never shift into the highest gear, and therefore your engine will be running faster than necessary and burning more fuel. These days, you surely don't want to do that.

Merritt

My experience is mostly with 5 speeds manuals, but in practice I find with my Mom's Camry I get better mileage around town with the OD OFF. On the highway it's better on.

Overdrive lowers engine rpm, it is a higher gear. A slower running motor- rpm, lasts longer, that is why diesels last. A motor that turns half the rpm say 1800vs 3600 lasts 4 times as long. I leave mine on. Overdrive will not be active till the motor reaces 110-120f. It also improves highway mileage, esentaily it is just a 4th gear, but one that will activate at even low engine speeds. Leave it on, the transmission automaticly does whatever you need. I imagine for moutains or heavy loads you may want it off, but normaly it is left on.

The one in my Tercel kicked in at 35MPH! The only problem was, when you turned it OFF, you were stuck in Third gear. It would only go into 4th with the OD on, and then lock-up came way too early.

In the '86 Camry, we get better mileage with it OFF for general around-town driving. It shift into OD about 37 MPH, not good for around town.

Simplest way to think of it is that you have an automatic transmission, and it will automatically select the right gears for you. Just put the car in Drive and go. Do not turn overdrive off. Your four speed transmission is acutally like a five speed. Automatic Transaxles, 1st, 2nd, 3rd, 4 th. Trans, Gear, Gear, Gear, Gear. Model, Ratio, Ratio, Ratio, Ratio. A140E, 2.810, 1.549, 1.000, 0.706 See the last gear ratio is less than "one" so technically it is "overdriven". The reason I say it operates like a 5 speed is that you also have torque converter lock up which links the flywheel to the transmission input shaft like a manual transmission with the clutch engaged. If you're climbing a hill in overdrive and observe the tachometer, first the torque converter lockup releases and the tach needle moves slightly, (almost like going from fifth gear to fourth), the if the hill steepens or you press the gas pedal further, at speeds under apx.