Overdrive on a Truck

  1. I may be lazy looker but this seemed like the best forum for this question. i just hope there are car people here.


    i have a 1993 GMC sierra V6 truck (dark blue, in case you were wondering). On the "shifter" i have the option to drive in "drive" or "overdrive."

    Now i don't really understand the difference too well. All the sites i have read to a rather poor job of explaining it: usually they use words that i don't understand or they are too make/model-specific for it to apply to me.

    on my way to school every morning, i drive 4 minutes at about 60 mph and roughly 4 minutes at 45-55. The road is relatively straight with little to no traffic or stoplights until the very last minute or so.

    From what I have understood, overdrive actually REDUCES the work done by the engine (in terms of RPMs) and INCREASES the gas mileage. Could someone outline what is going on AND tell me when (if at all) i should use OD (including in other situations). gas costs $2.65 and my truck is 12 years old...I need to become as efficient as possible.
     
  2. jcsd
  3. EnumaElish

    EnumaElish 2,481
    Science Advisor
    Homework Helper

    I always thought OD is the equivalent of the manual 4th gear. But I am not a mechanic. Have thought about calling a car talk radio show and asking your question there?
     
  4. This should be posted in engineering
     
  5. dduardo

    dduardo 1,919
    Staff Emeritus

    *Moved into General Engineering*
     
  6. Gas mileage decreases rapidly at speeds above 60 mph. I use overdrive when i'm cruse'in at 70+ {which is often}When you use overdrive gearing, your car's engine speed goes down. This saves gas and reduces engine wear.
     
  7. Your tranny shifts into a higher, larger gear, which reduces you engine rpm, increases fuel milage and hence is called 'overdrive'.

    Regards,

    Nenad
     
  8. so how should i be using overdrive? do i need it on my daily 10 minute commute to school, or only when i get going on a highway for half an hour? should i be shifting from D to OD while i am driving (i understand shifting between certain gears while pressing the gas can damage the transmission...maybe not in this case though).

    the car is an automatic by the way.

    thanks for the help.
     
  9. Basic operation of automatic transmission cars

    The question more appropriately might be "When should I not use overdrive?". In an automatic, the car selects the appropriate gear-ratios based on data such as engine vacuum. You should always drive an automatic in overdrive, no matter the speed, unless conditions warrant otherwise. This allows the car to select from the full range of gear-ratio options. Shifting down to drive restricts the range the car is allowed to select from. If you happened to be climbing a moderate hill and the car was shifting between drive and overdrive, you might want to temporarily shift it into drive so the car stops hunting between gears. If you happened to be climbing a hill while towing a 5,000 lb trailer, you might want to temporarily shift into a gear lower than drive.

    Also, some people like to shift down when descending hills. Shifting from overdrive to drive will provide a moderate amount of engine braking at typical freeway speeds.
     
    Last edited: Aug 20, 2005
  10. The etymology of 'overdrive'

    Steeper, actually.


    "Drive" refers to direct drive, which means a transmission ratio of 1:1, which means the output of the transmission is spinning at the same rate as the input. Most of the gears in a car are underdrive gears, which means they are below 1:1 (shallower or flatter than direct drive) and allow the engine to spin faster than the output of the transmission, and for years past the top gears on cars were typically 1:1 ratio, which is to say they were direct drive.

    An overdrive gear is a recent invention that is even steeper than a drive gear, and hence provides a gear ratio of greater than 1:1. Some high-end cars, such as the modern Corvette, have two overdrive gears. See the gear-ratio table here:
    corvettesbyrickdaniel.com/C6SPECTS.html

    According to that table, the two C6 6-speed transmission options provide 3 underdriven gears (gears one through three), one drive gear (fourth gear), and two overdrive gears (gears five and six).
     
  11. I don't see what is wrong with my statement that your engine RPM's decrease when you shift into overdrive. You stated yourself that the ratio mover from a higher one to 1:1.

    Regards,

    Nenad
     
  12. I wouldn't call it recent. Overdrive has been around for many years. I am not referring to aftermarket add-ons either.
     
  13. alright so i should basically always keep the truck in overdrive except:

    when climbing a long steep hill and it is switching gears erraticly

    when i am towing/have bed full of rocks and am climbing a rather steep hill

    thanks for all the help guys. my truck and i thank you.
     
  14. You might want to ask someone more in the know about this. Ford AOD's(not AOD-E's) have a weak OD band so it's actually better to drive them in D while in the city and only go into OD when driving for an extended time greater than 45. Older VW auto's had a similar problem. Actually, there are a lot of auto's out there that die prematurely due to weak OD servos. Do you know what type of trans you have(I'm guessing a 700-R4)? If you do have a 700 then OD all the time is probably a safe bet.

    I don't deal with GM's that often so I can't say off the top of my head what the best strategy is but simply saying OD always is wrong because there are transmissions out there that last longer if you avoid OD.

    My 2 cents. Good luck.
     
  15. Q_Goest

    Q_Goest 2,949
    Science Advisor
    Homework Helper
    Gold Member

    An automatic transmission in a car or truck has a "torque converter" which is essentially a hydraulic pump and motor combination. It allows the input and output shafts to differ in RPM slightly, so that one can idle in gear at a traffic light or change gears automatically. But it does this at a cost, the cost being lower efficiency:
    The overdrive feature locks the input and output shafts so this loss is eliminated and you can get higher gas milage.
    Ref: Louisianna State University
     
  16. Cliff_J

    Cliff_J 789
    Science Advisor

    faust9 - the 700R4 had plenty of improvements made to it over the years, the 89+ seem more reliable with an improved pump but they were never as good as the 4L60E. And even with that, my best friend from high school went through three 4L60E rebuilds in his regular cab 2wd S10 with the 122HP 4cyl and the most he towed was a jet ski! It just seemed the mountains going midwest-west coast killed it without any other effort needed. Its not like those GN guys who have done miracles with the little 200R4 metric tranny haven't shown how durable things can be made for short-term duty, but most engineers would see the impractical price that comes with it and how many of those cars are driven in traffic much less go 100,000+ miles?

    While on the topic though, I think it was Motor Trend who interviewed some NYC cab drivers about what they needed to do to their cars. Hard to believe a Crown Vic or Caprice could go 600,000 miles on 3 trannies and 2 engines, but there was more than one example of guys making it 350,000 on one tranny, in NYC! Reminds me of a Mad magazine sketch about a manufacturer who has a customer that bought their product 30 years ago and never had a problem with it buying it to find out what they did wrong to make it last so long without needing a replacement. :smile:

    For the OP, since you are not sitting in traffic you have really nothing to worry about. Heat is generally what kills an automatic transmission, and driving down the road the cooling should be plenty adequate for long life. Your mileage will improve a lot though, the 30% reduction in engine RPM will make a big difference in the overall mileage. And you could emprically test it too, try one week without OD and compare.

    You should make sure other things are up to spec too, check the air filter and tire pressures kind of thing. Even an O2 sensor can make a big difference, when they go bad on my TransAm the mileage goes from 28/20MPG to something like 20/16MPG and that is a shocker when you're driving like granny and the needle still flies towards empty! Luckily I have 4 O2 sensors to go bad (1996) and they are near impossible to replace since the crowsfoot wrench can't break them loose, so the garage sings with expletives once every couple years or so...
     
  17. Overdrive is any gear ratio beyond that of direct drive, which in turn is 1:1. The lockup feature is independent of the overdrive feature.
     
  18. What overdrive is and is not

    Allowing relative decrease in engine RPMs is not what defines an overdrive gear.

    Your engine RPMs decrease when you shift into second gear from first gear. That does not make second gear an overdrive gear. Only having a gear ratio beyond that of drive, 1:1, makes a given gear an overdrive gear.


    Probably not, since 1:1 is defined as direct drive, not overdrive.
     
  19. Danger

    Danger 9,879
    Gold Member

    Thanks for pointing that out; some of the foregoing posts had me pulling my hair out. I remember that when I first started into the street scene, there was an aftermarket unit called the Honodrive (sp?) that was installed right behind the tranny and gave an extra shift option. Also, 4th gear in my '72 Roadrunner (A833 tranny) is a .73:1 overdrive rather than direct drive. My former friend from the old days actually experimented with mounting a 3-speed manual tranny backward in line with the main tranny to give multiple overdrive ratios, but trying to clutch the thing proved a little too impractical. :rolleyes:
     
  20. That reminds me of the street racers who put an extra (rear wheel drive) engine and transmission in the back of a front-wheel drive car -- to make a sleeper. In the one I read about I recall that at least one of the two trannies was an automatic.
     
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