Question about overdrive

[supersnax]

Hi. I just bought an '04 and I love it. One question I have is about the overdrive on/off switch. The owner's manual says to "use this feature to increase your gas mileage" but it DOESN'T say if your mileage iwll be higher with it on or off!Anyone know the answer?Thanks

[tcam]

your mileage will always be higher with the overdrive on. however if your ever wanting more power, then you should turn it off.

[GrayFox]

just leave it on... all overdrive is 4th gear.I sometimes turn it off when driving on hills or going onto the interstate but 98% of the time there's no real reason to have it off welcome, BTW

[tribalman]

i have found it not to be just a 4th gear. it changes how the engine runs, slightly. when over drive is on (light on the dash is off) and i don't accelerate smoothly, the car doesn't lurch when accelerating. but if i accelerate fast with it, it does. plus it seems to lag a bit when you try and punch it, which i need to when getting onto highways. but it doesn't do this at all when OD is off. it just accelerates like a normal car. also, i stop and go traffic where you move about 5 feet then stop again, the car will once again lag in giving you acceleration and kind of lurch into motion giving you more speed than you need requiring a hard stop. and at that point the engine will idle, then after 1/2 second it will for some reason give power back to the wheels and lurch you forward a bit. none of this happens when OD is off.

[GrayFox]

http://forums.genvibe.com/zerothread?id=15765

[ajconover]

Yeah, "Overdrive Disable" may very well change the shift points (spacing things out a bit more) as well as preventing entry into 4th gear.I've noticed that modern electronically controlled automatic transmissions seem to shift themselves into the highest gear when you let your foot completely off the gas (depending on speed).... basically, it's so that when you are coasting, the drive wheels transfer less energy back to the transmission, torque converter, and engine. The effect can be replicated in a manual... if you are in 3rd gear for example, and take your foot completely off the gas, you get a lot more "engine braking" than you would if you were in 5th gear. I remember Saturn advertising a "smart" auto transmission (I don't know if they still use it) that would sense if you were coasting downhill and provide engine braking automatically.My guess is the "lag" and "lurch" you feel with overdrive enabled is the car trying to accelerate in 4th gear, then realizing that the RPMs don't match the throttle position (a momentary "bogging" down), so there is a sudden down-shift with the accelerator heavily depressed. When accelerating from 3rd with overdrive disabled, the effect is much less pronounced.Anyway... I have no real technical knowledge of the process, but I've noticed virtually every automatic I've ever driven (seemingly) behaving in that exact same way. My Lumina does the same thing... it doesn't have a button to disable overdrive, I just do it the old fashion way... by just moving the column shifter into "3" as opposed to "D". I do this on hilly terrain and in slow moving traffic. (My old '84 Buick actually had "D" on the indicator for regular non-overdrive and a "D" in a circle to indicate overdrive.)

[special_ops9]

Well this is how I see overdrive, its a different unit in a lot of trucks and earlier cars but I'm sure its not in out transaxles. But it is independent from the normal 4 or 5 gears and when off provides a 1:1 ratio between the transmission and the wheels when on it makes a slightly higher ratio (the wheels spin a little faster). So the gears in increasing ratios are actually 1, 1OD, 2, 2OD, 3, 3OD, 4, and 4OD. I always take it off when I get next to something I think I can beat and turn it off at about 70mph because the torque drops out when the engine is revving that high. Top speed with it off is 95ish and the engine is sitting at like 5500rpm on the base.

[NewNeptune]

I think you're thinking of the torque converter clutch. On the Vibe, turning off overdrive locks out 4th gear. Overdrive is synonymous with 4th gear (at least, on every car I've driven and read about).The lurching that you feel is the torque converter clutch unlocking to up the RPMs and provide you with more power, without actually changing gears in the transmission. The reason you don't feel it in 3rd is that it (in my experience) doesn't fully lock the torque converter clutch in 3rd gear, so you don't feel it unlocking when you lay into the gas.

[Whelan]

Overdrive simply keeps you in a gear longer and is faster to downshift to a lower gear if need be. It also locks out the 4th gear on the Trix. Give it a shot, get up to 80 or 90mph with OD off and you will notice the car get up to redline and still not shift.