Autotransmission gear question

Discussion in 'Motorized Vehicles' started by Phillip, Mar 3, 2010.

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  1. Phillip

    Phillip Phillip J. Fry

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    Since I live in a relatively flat region I barely drive in anything besides D or in the case of the car in question D4.
    So, I don't know much about using gears to drive an auto trans.
    My question is, what is the purpose of D3?
    I know it inhibits the trans from shifting into 3rd.
    Are there any other effects? such as shift times, ect.
    Also what if any would be the purpose of driving in D3?

    Thanks in advance.

    Fry
     
  2. Syndrome

    Syndrome Torque Matters

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    It only allows the transmission to shift into gears 1, 2, and 3. It disables gear 4, which means it will keep your RPM's higher which is where your power is in a typical gas vehicle.

    Same story for D2, and 1(sometimes called L). Its for going up mountains or while passing. You will get MUCH better mileage if you handle the downshifting manually versus stomping down on the accelerator to get it to down shift.

    And yes, with some vehicles it makes it so the other gears will hold their RPM's longer, but this doesn't apply to all.
     
  3. MAA83

    MAA83 Notebook Evangelist

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    Generally, for towing or driving up hills. Since it restricts the higher "cruising" gears it keeps your engine in the higher rpm's, presumably where your engine has more torque. Use it properly though. Torque converters in auto transmissions break when people try to drive their auto like a manual.
     
  4. Syndrome

    Syndrome Torque Matters

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    Not using it can also break your transmission. Just simply the concept behind how they automatically downshift puts tons of wear on them. You press the accelerator down, increasing the RPM of the motor, increasing the torque, then it shifts. Shifting in the middle of that torque isn't healthy.
     
  5. Trottel

    Trottel Notebook Virtuoso

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    I'm not really sure what you mean by this.

    This is true. Manually selecting gears (upshifting and downshifting using a 'manual' mode) in a traditional automatic transmission puts more wear on it than letting it shift automatically. When you go to D3 or whatever that doesn't hurt the transmission because all it is doing is disallowing it from going into its fourth gear.
     
  6. Syndrome

    Syndrome Torque Matters

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    I mean you will get better mileage by downshifting to 3rd before the transmission does it by its self. Because like I said before, you waste fuel by letting the engine rev to higher RPMs, then waste all that revving and shift. I have tested this theory and proved it(in a toyota Matrix, Not all transmissions are the same).

    Basically I shift into 3rd before I begin to go up the hill, I it at a moment I can keep the RPM's in my control, along with the power.
     
  7. EntityX

    EntityX Notebook Evangelist

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    If you want to drive an automatic like a manual, why not just get a car with a manual transmission? IMO messing around with automatics like that is likely to cause more wear and tear, that is unless your car is designed to shift like that, like a sporty automatic like an M3.
     
  8. Trottel

    Trottel Notebook Virtuoso

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    What do you mean letting it rev to high rpm's? If the engine gets to high rpm's it will not downshift, as that will mean the lower gear and speed of the car would make the engine rev up past the rev limiter. The car will rev a teeny tiny bit as you push the accelerator pedal before the throttle is open enough to downshift the engine, at which point the engine will rev much higher, so any increase in rpms before downshifting would not be any bit of a bad thing. You are trying to accelerate/increase the engine's power when you press the gas pedal, and that is what it is doing. Any increase in rpm's is because of this and is working towards your goal, not wasting fuel.
    How could you possibly test this? Doing this thousands of times in a row for each way? I can't possibly believe you.
    Okay, but if you downshift too soon, your rpm's go higher than they would be if fourth gear was selected, thereby wasting unnecessary fuel as the engine is less efficient at higher rpms, not that this would really be a measurable fraction of the fuel in your gas tank.
     
  9. houstoned

    houstoned Yoga Pants Connoisseur.

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    fyi: normal auto trannys with sport shift, or similar features, ARE NOT the same as a SMG/DSG/etc system found in a later model m3.
     
  10. EntityX

    EntityX Notebook Evangelist

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    I realize that. But all those types of shifting are designed to be shifted during regular use, however the d2, d3 etc kind are only designed to be used during special circumstances, i.e. towing or going down/up hill.
     
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