Overdrive question

I am a bit confused as to the way overdrive works.

They way I understand it to be, the overdrive gear (4th gear on most cars) has a lower ratio of spin (less than 1:1), so the engine spins slower, and therefore better gas mileage. However, someone recently told me that if you turn the overdrive off, the car gets better acceleration. But this would mean that the RPM at which each gear switches is changed. That right there doesn't seem right to me.

I think I'm right, but I'm not completely sure. Could someone clear this up?

im not 100%, but i beleive this is how overdrive works:

when you accelerate and get to a comfortable speed (say 40mph) then the transmission goes to a higher gear to save gas. This is like going into 5/6 gear at 40 in a manual. It turns less RPM thus the engine doesn't work as hard and you get better gas mileage. but it's a double-edged sword: just as in a manual if you shift too early you get zip for torqe and if you're going 30 mph in 6th gear and you floor it, it does jack shit. thus turning overdrive off when you rest at 40mph it stays in the same gear and so when you decide to gun it, the torque is there. but you're also cruising at 40mph at about 3-4 rpm which is terrible on gas.

also another thing, the rpm at which the transmission shifts can be changed. on some transmissions i beleive depending on how hard you stomp the accelerator the higher RPM it goes to. I'm pretty sure my mothers van does this. If I merely step on it, it goes 1-2000 rpm before the redline and changes gears. but if i stomp on it, it redlines. this way if you need to pass someone real quick on the highway you can just gun

Thank you.....so turning the overdrive on or off shouldn't affect acceleration, right?

I always thought overdrive was a 1:1 ratio. That or a lockup torque convertor was used to make a mechanical connection through the transmission to achieve gas mileage of a manual.

one description of "overdrive" is simply a gear in which the tranny output shaft is spinning faster than the input shaft.

Originally posted by enginerd
I always thought overdrive was a 1:1 ratio. That or a lockup torque convertor was used to make a mechanical connection through the transmission to achieve gas mileage of a manual.

well regardless of gear ratio, the torque converter always wastes a little gas. The relationship between the wasted-energy and the speed of the car is inversly proportional on some level and thus the higher the speed the closer the torque converter is to not wasting any energy but it always does. But maybe you have a point, maybe it bypasses the torque converter, because it's only purpose is to low the torque going to the wheels enough so that the brake is able to stop the vehicle completely and rest, so when it's in motion at any time it's useless. I'm going to look into this it has me curious now, because bypassing the torque converter would also have positive acceleration effects as the torque has one less stop and less power is lost before it hits the wheels.

"By definition, an overdrive has a faster output speed than input speed. It's a speed increase -- the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission.

In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.


Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1
So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow."

So it was all of the above. It is a gear that allows the engine to spin at a low RPM yet be at high speeds (like using high gears in a manual for lesser speeds), and some cars also bypass the torque converter as well wasting less energy.

the automatic transmission is way too complex, stick with a std! :p

were you quoting an article from http://www.howstuffworks.com ?

Originally posted by ivymike1031
were you quoting an article from http://www.howstuffworks.com ?

yep. i read through the automatic transmission one before but its a difficult concept to understand and i brushed over the overdrive section so i thought i'd check back to see if they covered it and it was there.