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Overdrive function on auto transmission

Discussion in 'Off Topic [BG]' started by Vince S., Jul 8, 2005.


  1. Vince S.

    Vince S. Resident Former Bassist

    Jan 24, 2003
    Reading the car thread got me wondering: What does the overdrive function do? I've seen the button on our car's shift lever and the icon on the dash.

    Vince
     
  2. Something to do with downshifting when going uphill I think? Someone explained it to me years ago but I've forgotten since then. I usually drive a manual tranny.
     
  3. I believe overdrive is a higher gear than a standard tranmission's highest gear, which usually has a 1:1 ratio.

    When overdrive is engaged, the engine will have a lower revolution rate. It's mostly used for long distance cruising on an open highway in order to save fuel and wear & tear on the engine.

    Mike ;)
     
  4. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses, Hipshot products
    I got a new car (to me) a month ago and I was wondering the same thing. I was advised to simply not use it. First couple of days I did, and it gave me a little extra boost to get on the highway or pass cars - then it got stuck in the Overdrive Off position and at 60 mph the tachometer was at 5000. I had to stop and restart the car to reset it. I don't mess with it anymore.

    I still don't understand however how the overdrive "off" light is on, when it seems to be engaged??? If I drove with it off, which would seem like what one would want to do, the light would always be on, and I'd be racing the engine all the time. Maybe my new used car is screwed up?
     
  5. I just re-read my post.

    When I said a standard transmission, I meant any ordinary transmission be it manual or automatic.

    Mike. ;)
     
  6. Joe, if your engine was reving 5,000 @ 60 mph, it sounds like your tranny was stuck in 2nd gear.

    Where's Mike N? :cool:

    Mike
     
  7. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    Yep. You guys can speculate all you want, but when Mike N gets here, we'll know for sure what the Overdrive is all about. :D

    -Mike
     
  8. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses, Hipshot products
    Yeah - it pretty much seemed that way. My new car has me slightly nervous. Don't mean to hijack this thread, BUT - it has a button for electronic transmission control also. The button seems to do nothing. I was fiddling with both that, and the overdrive button. I have no idea why the car weirded out as I mentioned above, but I've since driven it about another 1000 miles and it hasn't happened again. 2 nights ago however the check engine light was on. Once again, I turn the car off, start it up - and the problem is gone.

    I have 4 more days that the car is under warranty.

    I'm thinking of driving to California and back in 4 days so if anything CAN go wrong, it will... NOW!
     
  9. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    Drive to Seattle and back. When you get to Seattle, I'll buy you a cup of coffee, and we can go to BassNW. It would be cool to meet you.

    -Mike
     
  10. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses, Hipshot products
    Thanks for the invite but I've got gigs tonight, tomorrow and monday. I wuzzzz just kiddin anyhow.
     
  11. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    Ahh, the life of a rock star. :)

    I kinda figured that, but it was worth a shot in case you were serious.

    -Mike
     
  12. xshawnxearthx

    xshawnxearthx

    Aug 23, 2004
    new jersey
    in our van we have an overdrive button and its just a bigger gear in the differential that makes the engine work less.

    thats how it was explained to me.


    on my car, when its engaged(by stepping on the pedal to the floor, no seriously) it drops the gear from say, 3 to 2 so its giving it more power.
     
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Overdrive has a ratio of higher than 1:1 and gives you somewhat better gas mileage, lower engine wear, et al, at the expense of some torque and pick-up. For just cruising along, leave it on. If you need to accelerate quickly and can't wait for a downshift, turn it off.
     
  14. Overdrive gives you a final gear ratio of less than 1:1 (like 0.9:1). It used to be a separate planetary gearbox between the transmission output shaft and the driveshaft. The O/D on-off button just doesn't let the transmission shift into this gearing. I have the button on my car, but I've never used it. I'm not sure what it would be for anyway.
     
  15. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    Michigan
    Basically, think of your automatic transmission as a manual transmission, with the car picking the gears for you, instead of you having to shift. In this case, the "overdrive gear" would be the final gear in the sequence. It turns your tires at the highest speed, but sacrifices some torque (in turn, the ability to accelerate is diminshed, etc.). The "O/D off" switch DISABLES your cars ability to shift into that highest gear. That means your car will stay in 3rd gear (as most automatic transmissions have 3 gears plus and over-drive gear) or 4th gear, on a newer 5 speed automatic transmission. This is useful for accelerating on the highway, as you have more torque available at the wheels in drive than in overdrive, and is also helpful when climbing hills, again due to the torque issue.

    Joe. Your car would probably not let you shift back into overdrive mode because automatic transmissions are set to shift automatically when certain engine speeds and vehicle velocities are met. You were in 3rd gear, and had exceeded the conditions necessary to cause the car to shift to OD (fourth speed), but had disabled that shift with the switch. It might damage the engine or transmission to shift at this point, so it wouldnt let you. I bet if you had slowed down to a speed where third gear would have been appropriate, hit the switch, and then proceeded to accelerate to a speed suitable for OD, it would have been fine.

    That is my limited knowledge of automatic transmissions and their workings. Please feel free to ask more questions, or correct me in my error. :)

    Regarding your check engine light, Joe, cars store the "codes" in their computers until they are reset manually, either by computer, or a manual procedure (often "turn the car on and off three times in 10 seconds"). The code generated with your light should still be in memory. Take it to autozone, they'll read the codes for free and tell you what it was. The fact that it has not created a code again is a good sign though. It'll prolly come back with some meaningless garble about manifold pressure or a temperature reading somewhere. Seeing as it only happened once, it was probably just a hiccup. Seeing as you only have a few days left on your warranty, best to get it checked. DONT get it checked at your dealership, as they will for sure tell you they didnt find anything. Good luck!
     
  16. 2005 Ford Mustang GT
    Gear, ratio
    1st 3.38:1
    2nd 2.00:1
    3rd 1.32:1
    4th 1.00:1
    5th 0.68:1
    Final drive ratio 3.55:1

    1st gear, the engine turns 3.38 times for every time the driveshaft turns once. You get up into 4th gear, and it's 1:1, essentially a straight connection. 5th gear, which is called overdrive, has the driveshaft turning faster than the engine. That's all overdrive means.

    The final drive ratio is the gearing of the rear differential. Multiply the two ratios and you get the engine:wheel ratio. 1st gear, it's 3.38x3.55 = 12:1. Therefore, even in overdrive, the engine is turning faster than the wheels are (2.414 times faster).

    On a 4-speed automatic, usually 4th gear is overdrive. Pressing the button basically turns it into a 3-speed, and keeps it from using 4th. Why would you do this? How about when you're driving up a long hill and it's shifting 3-4-3-4-3 and really annoying you? Just turn off overdrive and it stays in 3rd the whole time.
     
  17. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    Michigan
    Slightly more succinct than my definition, but I was trying to avoid actually having to look up the drive ratios for a car. :)
     
  18. But yours was up first! :cool:
     
  19. There's a lot of misinformation here, and Dan is the only one who's actually said what that button does. Normally the vehicle's transmission shifts into the overdrive (highest) gear when enough pressure builds up in the transmission fluid like with any other gear. Pressing that little button disables this gear, and prevents the car from going into overdrive. This is used iirc for towing things, and should NOT be enabled for normal driving as it kills your mileage in times when your car should be using overdrive.
     
  20. Joe Nerve

    Joe Nerve Supporting Member

    Oct 7, 2000
    New York City
    Endorsing artist: Musicman basses, Hipshot products
    Thanks Dan.