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Most people know that for old or even somewhat old cars, manual transmissions are more efficient. Recently automatic cars have become almost as efficient or even as efficient as manuals.

I was recently reading an article by Road & Track magazine that was comparing different sports cars, and it mentioned the fuel efficiencies of the manual versions versus the automatic versions, and actually listed the automatics as being more fuel efficient than manuals by about 2 mpg.

Are automatic transmissions now really more efficient than manuals? Is this for all cars or just high-performance cars?

If autos are now more fuel efficient than manuals then how have they managed to make this so?

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    There's a lot of variability in how manual gearboxes are treated; computer-controlled automatic gearboxes can take out some of the outliers, particularly at the high revs end. So it depends how you measure the efficiency. – Chris H Feb 26 '18 at 9:26
  • This question depends a lot on a proper definition of "efficiency". An automatic transmission will never know when in the future I want which gear. E.g., on my way home, there are some bumpers I usually heel+toe over, which, in short, means I brake and change into the right gear at the same time, blipping before I re-engage the clutch; so as soon as I am on/behind the bumper, I can start through. An automatic transmission cannot anticipate if I want to start through, slow down a bit, or want to come to a complete halt. – phresnel Feb 26 '18 at 9:49
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    You should bear in mind that most comparisons that say "A human is better than an automatic system" rely on the human being pretty good at their job. It wouldn't be at all surprising if automatic transmissions have been more fuel-efficient than average-to-poor drivers for a long time. – David Richerby Feb 26 '18 at 10:19
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    @phresnel A human indeed cannot be predicted. That is why the more sophisticated automatic transmissions always have another gear already engaged for you, ready and waiting behind the second clutch. Again i refer to DSG since that is the one i am most familiar with: en.wikipedia.org/wiki/Direct-shift_gearbox – MadMarky Feb 26 '18 at 15:17
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    @RichieFrame: Oh yes, in that variant, a sportive driver has a harder time beating the car. I would call that "semi-automatic" or "semi-manual", though. Such configuration is a bit like Formula 1 cars, where drivers manually clutch for the first gear, but during driving, they just use paddles. [let me google that ...] oh yeah, the official source (Formula1.com) says: "Formula 1 cars use highly sophisticated semi-automatic". – phresnel Feb 26 '18 at 15:34
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Traditionally...

The major drawbacks of automatic transmissions were:

  • parasitic losses in the torque converter, something which manual transmissions don't have.

  • fewer gears, so a given engine was more likely to be in its sweet spot with a 5-speed manual than a 4-speed automatic during regular operation.

  • gear selection logic which was inferior to well-trained human drivers


Over time...

Transmission manufacturers have worked out ways to reduce the impact of these drawbacks, including:

  • Direct-drive to lock out the torque converter to prevent losses
  • Adding more gears helps keep the engine in the right rev range for optimal fuel economy when the conditions allow for it (8-speeds are quite commonplace nowadays, even 10-speeds)
  • Quicker and more intelligent gear-selection logic that can handle a wide variety of driving scenarios.

For further reading: see this reddit comment, which does a good job of summarizing the answer to this question.

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    Presumably a CVT would fall into the second category? – Michael Feb 26 '18 at 21:56
  • @Michael yes, the big draw of CVTs lies in their ability to continuously vary gear ratios, enabling engine RPMs to stay constant across a wide range of vehicle speeds. – Zaid Feb 27 '18 at 7:06
  • Another major drawback was that automatic transmissions tend to by a few hundred pounds heavier than a manual transmission, correct? – Adonalsium Feb 27 '18 at 15:58
  • @Adonalsium I believe they still are – Zaid Feb 28 '18 at 9:07
  • CVTs are considerably lighter than "traditional automatics". @Zaid you should just quote the relevant text so one doesn't have to dig and cause links die yadayada – finleyarcher Feb 28 '18 at 17:31
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A lot of new automatic transmissions (e.g. Volkswagen DSG) arent actually auto transmissions, at least not in the traditional sense. They have a normal computer-controlled dry clutch (well two actually) so they do not need to move around a lot of transmission fluid like traditional automatic transmissions do. Also they have a lot of gears (7, 8 or even 9 speeds like the Mercedes 9G-Tronic) and upshift quickly when cruising to have better gas mileage. That part explains why modern automatic transmissions often are even more fuel efficient than manual transmissions.

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    Not only the expensive and sophisticated DSGs, even the simple Fiat dualogic is fuels efficient and also CVTs, common to all is not using a torque converter. – Rsf Feb 26 '18 at 14:22
  • So, are they only more efficient, because the comparison is between inherently unequal options? Is there a similarily layed-out manual transmission available? Would it be more efficient? :) – Alexander Kosubek Feb 26 '18 at 15:42
  • Most of the DSG transmissions are wet clutch type (especially for high torque applications), like in motorcycles. – oryades Feb 27 '18 at 7:16
  • @AlexanderKosubek Fiat's dualogic is a somewhat good comparison since it has more or less the same number of gears as manuals. this list shows that it is more or less equal to manual transmission. – Rsf Feb 27 '18 at 15:43
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With a greater number of gears in the autobox and with a lock-up function called more frequently for the fluid flywheel.

Then add a better computer based control system for moving between gears : improved algorithm and faster change control.

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