How does overdrive actually work?

[banda536]

I have a 98 gs. I dont know how the overdrive actually works. I just leave it on. How does it actually work? How are you supposed to use it?

[JoeWagon]

overdrive is a gear ratio of less than 1:1 http://auto.howstuffworks.com/automa...nsmission3.htm

[will2shower]

when you leave OD on it just tells the computer that when you are cruising at a moderate speed while your car is in 4th gear lets say... that instead of keeping the car in 4th gear it will shift the car into a 5th gear.. so the only time that OD on is when you would normally be in your highest gear. i think....

[EclipseRST]

i dont think that is right cause i know on the automatic eclipses' that with the overdrive on it will kick in around 40mph and drop the RPMs about a grand...

[EclipseRST]

oh and i believe the auto tranny only has 4 gears... L, 2, D, OD

[will2shower]

that is pretty much what im saying, going 40 down the road the transmission is proubably in its highest gear. so the next available gear would be OD

[kjewer1]

Pretty close, but on an auto tranny overdrive is 4th. On a 5spd it is 5th. The problem is that 10 years ago people figured out that OD comes on right around 45 mph in most cars, and thats where people spend the most time cruisng. So it would kick on and off repeatedly, wearing out the clutch bands for 4th too fast. So now for people that dont use the highway they can just turn it off. The other day in the wifes car, the computer actually saw that it was shifting in and out of OD too much and flashed the OD light, presumably telling me to turn it off. Just because of all the hills on that highway. So modern cars are a little smarter than the old ones.

As mentioned above, anything higher (numerically lower) than 1:1 is considered an overdrive. A common one is .8:1. A more typical gear ratio might be 3:1. So for every 3 revolutions the motor makes, the tranny output makes 1. With the OD ratio, for every .8 turns of the motor the tranny output does 1 turn. It turns faster than the motor, and thus is "overdriven." Now, while gear ratios that are still "underdrive" there is torque multiplication, as we have discussed here recently. But in over drive there is the opposite, torque reduction. But since you are already at highway speeds, you only need enough torque to maintain your speed against the wind and gravity. The whole point is simply to slow the motor down to consume less fuel.

Its also important to mention that we havent yet mentioned the final drive of the car. DSms are somewhere around 4:1. This would be the equivilent of the rear end gear ratio in the RWD muscle cars. 3.73s, 4.11s, etc. So the motor is always turning faster than final output, even when the tranny is in overdrive.

[banda536]

So i should just leave it on all the time? or just leave it on the freeway? or when im in high speed?

[kjewer1]

Just leave it on, unless you rarely ever go over 45-50 mph. For 99% of the population I would say just leave it on. You can always turn it off, the only risk is that if you forget about and are moving along at a good clip, you will get less than ideal fuel economy...

[RiceRocket18]

you mean leave it OFF unless you're going 45+ on the freeway.

so does it affect your top speed? does the car perform faster with the O/D OFF?

[JoeWagon]

Overdrive is a higher gear, so yes... you will accelerate faster NOT in O/D. That's why overdrive turns off if you floor it. If you are going top speed, you are going to be close to WOT, so O/D will be off anyway. Damn this is an old post.

[RiceRocket18]

WOT?

so basically speed/performance wise, i should keep the O/D "OFF" at all times then.

does that mean my A/T will only use 3 gears with O/D off if the 4th is overdrive

i know it's an old post. i just didn't wanna create a new one and start al over. i was playing around with the O/D feature today and didnt know what to do with it .

[EclipseRST]

WOT = wide open throttle

yes keep O/D off unless your cruising on the highway!

[RiceRocket18]

thanks for the help guys... just went out for a test drive with OD "OFF". it definitely felt a lot more powerful. 17.5 PSI LOL man!.. one day my T25's gonna crap out!!. oh well.. i'll just get a super 16G.