Question about overdrive button

[VinceSez]

I've searched through the forums and read about the overdrive button, but I wanted to get something straight. I should only turn O/D off when I'm in the city, but when I'm on the highway for example, I need to turn it on? The reason why I'm asking this is because when I first bought my car in Feb, the O/D button was off. Am I hurting my engine with it off?

Thanks in advance

[masticatingcow]

Keeping the O/D button ON helps your gas mileage by opening your final transmission gear (4th) for the engine. You should only turn overdrive OFF when you've got a lot of weight in the car and/or you need the extra power, like when driving up steep hills.

[Red coupe]

If you have heard it recomened to turn it off for city driving its because its lots of automatic transmitions will shift into 4th at about around the speed of some faster city streets.This could mean premature wear of the clutchs for overdrive... The transmition uses a serious of friciton plates that are hydrolicly controlled, were most gears have 5-6 sets of clutch discs to apply them OD will typically have less (the toyotas ive seen had somewhere closer to 2 sets). Point being the clutch for 4th gear is most likely weaker then the others, with the assumtion it will handle relitivly low torque loads, and be aplied much less often then other gears.

[VinceSez]

ok, off it is then when on the highway, and on when on the city streets.....

[slc240sxse]

Well not just then, but as mentioned previously, when you need a forced down shift, such as when you climb a hill, have a heavy load, or need to pass some POS Honda with a 40lb wing on the back.

Christ I hate Hondas.

[Red coupe]

no no on when its on the highway....if you turn overdrive off you car will not be able to go into its highest gear, it would be kinda like staying in 4th with a manual. For most longevity of your transition leave it off around town and turn it on when your doing extended sessions of 50+ driving. Also with it in overdrive the detent switch on the throttle should still work to force a downshift at WOT...but like Masticating cow said also switch it off for those drive up steep mountain roads, even though it should downshift when engine load overcomes vehicle speed (throttle pressure rising higher then governor pressure in the throttle body)

[Bosrudorfer]

no no on when its on the highway....if you turn overdrive off you car will not be able to go into its highest gear, it would be kinda like staying in 4th with a manual. For most longevity of your transition leave it off around town and turn it on when your doing extended sessions of 50+ driving. Also with it in overdrive the detent switch on the throttle should still work to force a downshift at WOT...but like Masticating cow said also switch it off for those drive up steep mountain roads, even though it should downshift when engine load overcomes vehicle speed (throttle pressure rising higher then governor pressure in the throttle body)

How would this apply to a KA-T automatic?

I would turn it off when driving in the city, turn it on when cruising the highway (not building boost) but what about say hidden roads where I would like to hit some top speeds (say 75-100mph)? Would I be able to do this with overdrive off?

Thanks!

[Red coupe]

Overdrive is simply that, an additional gear set there to provide a gear ratio less then 1:1 (IE driveshaft faster then engine speed).

The reason I said to leave it off around town is that durring normal throttle 50ish is about when the transmission shifts up into overdrive.

Again like I said before its just another gear(actualy another gear ratio, autos are different in that they use two sets of gears for all the normal gear ranges, with a third set activated for overdrive) Each gear range in an auto is chosen by activating different sets of clutches. Depending on intended use of the gear(torque being transmitted, conditions under which it is applied, ect) different numbers of clutch plates are used. Gears like first may have some where around 6 fiction plates (been a while but quite a few for gears that deal with more torque) but since O/D clutch is always applied under such weak torque loads it has significantly less clutch material (maybe 2 clutch plates)

Durring around town driving the transmission is having to constantly shift in and out of its overdrive gear. This causes exessive wear and could EVENTUALY(long term) cause overdrive to wear out prematurely...

Its not a huge problem and isn't gonna happen over a day or two, really more of those longer term concerns so that no one part of the transmission wears out before the rest of it needs a rebuild.

AS FAR AS WHAT I WOULD RECOMMEND FOR YOUR KA-TOn back roads I would leave it off....Again while being in overdrive may signal to the computer that you are driving more conservatively(The computer may take it into account as far as shifting slightly sooner/softer but I really dont know honestly) its MAINLY just a 5th gear.....If your not running out of RPM's with it off, or trying to get better gasmilage while driving hard() I see no reason to have it on, as it will just make the trans shift up more and autos do that to much anyways....