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Thread: Why usually most automatic cars are tall-geared?

  1. #1
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    Why usually most automatic cars are tall-geared?

    Why usually most automatic cars have taller gearing than the manual cars?
    This is particularly the case with older automatic cars.

  2. #2
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    Quote Originally Posted by karabiner98k View Post
    Why usually most automatic cars have taller gearing than the manual cars?
    This is particularly the case with older automatic cars.
    to compensate for the mechanical and frictional losses, which result in higher fuel consumption?
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  3. #3
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    Also with older cars there's usually fewer gears.
    Life's too short to drive bad cars.

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    Quote Originally Posted by henk4 View Post
    to compensate for the mechanical and frictional losses, which result in higher fuel consumption?
    Thanks. Any other ideas?

  5. #5
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    Fewer gears means you have to use the available ratios more broadly.
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    Quote Originally Posted by IBrake4Rainbows View Post
    Fewer gears means you have to use the available ratios more broadly.
    Yes, that's true but i have seen many cases in which the ratios of the automatic are still taller even if the number of gears is the same.
    I mean overall ratios (gear ratio x final drive ratio) of the automatic transmissions are most of the time taller than manual transmissions.

  7. #7
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    What do you mean by taller geared? Do you mean top gear or first gear.

    Traditionally autos have a taller first gear because the torque converter increases the off idle torque of the motor. So instead of needing a low first gear to get going from a stop, the TC multiplies the limited off idle torque of the motor such that the engine doesn't stall with a taller first gear ratio.

    As for the top gear, I've only noticed taller top gears on recent 6 speed autos. In that case they are doing a double overdrive like the Corvette has done for years. On the Corvette, people moaned that the tall top gear made the top gear acceleration really slow and thus even with 400hp a C6 required a downshift to get the same passing power as a V6 Accord. Well with an automatic the downshift is easy. The car does it for you so why not have a really tall top gear for good mileage and then a somewhat lower gear that is still tall enough that you don't notice your Civic just downshifted to go up a hill but low enough so that the Civic can make it up the hill with some feeling of power. Since the autos shift for you why not have the really tall top gear.

    BTW, I'm not sure what the mechanical losses are in a modern auto but thanks to lock up torque converters I don't think they are much if any worse than manuals as you are moving. They are worse at traffic lights as the engine is under load vs being truly unloaded with a manual or DSG box.

  8. #8
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    Quote Originally Posted by culver View Post
    What do you mean by taller geared? Do you mean top gear or first gear.
    I don't mean any gear alone, i mean the OVERALL RATIO.
    Take for example Peugeot 206 1.6 16V (TU5) with AL4 transmission with the following ratios:

    2.725
    1.499
    1.000
    0.711
    Final drive ratio = 3.65

    And here are the overall ratios:
    9.94
    5.47
    3.65
    2.59

    If you look at the overall ratios, you will see that they are insanely tall for a tiny, torque-less engine like TU5 which has only 147nm@4000RPM

    Now look at the following ratios for the Mercedes 190E W201 with 4-speed manual transmission:
    3.91
    2.32
    1.42
    1.00
    Final drive ratio = 3.23

    Overall ratios:
    12.62
    7.49
    4.58
    3.23

    190E has much better torque curve in low revs and it has 178nm@3500RPM but the ratios are much better than the insanely tall ratios of 206 which has a tiny, torque-less engine which peaks at 4000RPM.

    Comparison of overall ratios:

    190E 206
    12.62 9.94
    7.49 5.47
    4.58 3.65
    3.23 2.59

    My question is that why the ratios of 206 should be much taller than 190E ratios despite the fact that 206 needs much shorter gearing to compensate for it's lack of torque at low revs?
    Last edited by karabiner98k; 11-28-2010 at 09:39 AM.

  9. #9
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    A 206 is a LOT less weight than a 190E and so doesn't need as much torque and they'll meet better fuel consumption tests.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  10. #10
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    Quote Originally Posted by Matra et Alpine View Post
    A 206 is a LOT less weight than a 190E and so doesn't need as much torque and they'll meet better fuel consumption tests.
    The truth is that 190E weighs between 1100-1140kg from 1982-1988 and 206 XT 1.6 16V Automatic weighs 1119kg.
    So, they are almost the same in terms of weight.
    Last edited by karabiner98k; 11-28-2010 at 02:00 PM.

  11. #11
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    Brother had the Cosworth 190E and it was a LOT lardier than that.
    200kgs more than the 206 you've cited.
    AND if you take the even lardier "Normal" variants with auto then I think you're up into 1700kgs
    If you're comparing older/lighter 190s then my bad BUT then what is the point ???
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    I doubt that even the six cylinder auto 190Es were up to 1700kg. 1500kg at most I would say.
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  13. #13
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    I would suggest we forget this comparison, and focus on a car (or cars) that has (have) an auto and manual, and the same number of gears.

  14. #14
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    ^^^ I was mixing memories a bit as I'm sure that was the full up weight. ( checked uk club and 1630 )
    So most the transmission can pull
    Still, stock kerb weights enoguh difference and as I'd also said bigger issue is fuel consumption tests imo
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  15. #15
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    Alright, just to help the conversation along I'll toss out example figures; we have two cars, same base model and model year, one manual -- TR3650 -- one automatic -- 5R55S. Generally the automatics dyno 1-4% lower than the manuals.

    TR3650
    1/ 3.38:1
    2/ 2.00:1
    3/ 1.32:1
    4/ 1.00:1
    5/ 0.68:1
    final drive/ 3.55:1

    5R55S
    1/ 3.25:1
    2/ 2.44:1
    3/ 1.55:1
    4/ 1.00:1
    5/ 0.75:1
    final drive/ 3.31:1

    No, no points for guessing make and model.
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