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>when automatic transmissions are consistently outperforming humans at fuel efficiency Do you have data to back this up? Unless you're driving a dual clutch "d

• sshanky 10y ago
  In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because it takes some fuel to keep the engine idling, where staying in gear allows the computer to completely cut fuel flow, as long as the forward motion is keeping the engine turning.
  • baq 10y ago
    can confirm with anecdata: in my car the computer reports 0.0 fuel consumption when coasting in gear but non-zero when in neutral/clutch depressed.
    • ldev 10y ago
      Simple understanding on how a car works - "wheels turn the engine" when in gear, otherwise fuel is consumed when in neutral..
      • justin66 10y ago
        Not so simple. Until recently, a standard ICE couldn't stop injecting fuel while coasting (unless you turned the car off, which brings other problems...). I remember when I was learning to drive being taught about engine braking (with an automatic) and the tradeoff between using up the brakes or burning fuel.
        • ygra 10y ago
          For every fuel-injected engine the ECU should cut injection when coasting. So "recently" in your case applies to carburettors which will behave as if the engine were idle and thus still feed fuel to the engine. I'm not sure 25 years ago counts as "recently".
          • justin66 10y ago
            > For every fuel-injected engine Well, I own a 911 that says otherwise. It's an older fuel injection system (Bosch K-Jetronic), but it's most certainly fuel injection. Porsche wasn't the only maker to use Bosch fuel injection. I just don't think DFCO (deceleration fuel cutoff) was a feature of most of the early electronic fuel injection systems. Perhaps it was standard by the time EFI made it to economy cars. I'm fighting a massive urge to go down this historical rabbit hole and read about it, but there's work to be done today.
    • ptaipale 10y ago
      Of course it works that way, but just for the note: what the car computer reports in dash is manipulated by software and it can be anything. It's not necessarily completely true.
    • PanMan 10y ago
      A counter point: In my previous car the screen showed 0.0 when idling, which obviously was false (as the engine was still running). It should probably show infinite instead. (km/l for 0 km).
      • _rav 10y ago
        Some cars show liters per hour when stationary, which always seemed logical to me.
      • petre 10y ago
        Or change to l/h when you're stationary or with the clutch disengaged.
    • kbart 10y ago
      another anecdata: I once drove a car that reported negative fuel consumption when driving down the hill. EDIT: no, it was neither electric nor hybrid car, but some old petrol car with manual transmission (Saab 900 or similar).
      • mannykannot 10y ago
        A hybrid?
      • TeMPOraL 10y ago
        Electric cars generally do that ;).
      • deleted 10y ago
        [deleted]
  • nostrademons 10y ago
    How modern is modern? I could believe this of my 2009 Honda Fit, but I'm fairly certain that my dad's '92 Civic, '99 Civic, and '92 Accord got better mileage when coasting with the clutch disengaged.
    • dogma1138 10y ago
      92 was 25 years ago, this isn't a modern car, it should not be on the road if you care about economy and road safety.
      • nostrademons 10y ago
        Oh, it isn't on the road, my dad totaled it in 2008. It was, however, what I drove for my first 3 years in the workforce, so I got a pretty good feel for how to drive it economically.
    • jfitbsidbfb 10y ago
      My 1998 Saturn noticeably cut fuel on deceleration, it was easy to tell since the exhaust system was perpetually breaking
      • bborud 10y ago
        Come to think of it: the car I drove in 1988 cut fuel when coasting and it had a rather unrefined ECU mapping that would cause somewhat hard transitions from the fuel-cut state to feeding the engine go-juice.
    • semi-extrinsic 10y ago
      Basically any car that has electronic fuel injection does this. So at least the '99 Civic definitely does it.
    • Ntrails 10y ago
      Think about what is required to turn the engine over and prevent stalling. -When the clutch is engaged whilst going down hill the wheels turn the engine. No Fuel Required. -When the clutch is disengaged the engine has to turn itself over - which needs fuel. *Disclaimer, i'm sure carbourettas (sp?) could well be doing weird ass things so whilst conceptually the above holds they may well be different.
  • yason 10y ago
    This used to be the case with fuel injected engines from 80's to 00's but because of new regulations current ECUs might, again, inject fuel while coasting just to keep the catalytic converter warm and operational. The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :)
    • Retric 10y ago
      Smog is a real health issue, and US city's are vastly better now. So, IMO it's well worth a tiny bit more global warming.
      • gregors 10y ago
        Check out what's going to go into Chinese cars ..... https://www.youtube.com/watch?v=S3cFfM3r510 https://www.youtube.com/watch?v=S3cFfM3r510
    • crottypeter 10y ago
      > The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :) That is what Volkswagen are being forced to do.
    • justin66 10y ago
      This is interesting. How do hybrids handle this? Use electricity to warm the converter?
      • morganvachon 10y ago
        This may be a bit too much information, but it's a good read if you're interested in the technology behind it: https://hal.archives-ouvertes.fr/hal-01062316/document https://hal.archives-ouvertes.fr/hal-01062316/document
    • mikepurvis 10y ago
      Emissions per mile, miles per gallon, capital cost. You can only optimize two of them.
  • 3poundkilograms 10y ago
    When you coast with engaged clutch, you will engine brake. Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train. Plus, more efficient cars still have to inject fuel. The electro-hydraulically controlled conventional gearbox of the Lupo 3L disengages the dry clutch while coasting for a reason. Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking. Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.
    • Yetanfou 10y ago
      When you coast with engaged clutch, you will engine brake True Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train It is a bit hard to understand where you're trying to go here but assuming that you state it takes fuel to 'counter the engine braking' you're wrong. Engine braking is just that, using the engine as a brake. The engine is used as a pump, air (and only air, no fuel is added) is compressed, the air heats up, this heat is dissipated by the cooling system. That is where the energy actually goes, as heat. Losses in the power train only add to the braking effect. This is also why it is a bad idea to use engine braking with a 2-stroke petrol engine: 2-strokes rely on oil mixed in with fuel for lubrication. When using the engine as a brake no - or hardly any - fuel is added to the incoming air while maintaining high engine speeds, starving the engine of lubrication.
      • Dylan16807 10y ago
        They're talking about the situation where you're coasting and don't want to brake.
      • 3poundkilograms 10y ago
        It's a bit hard to understand where you're trying to go here but assuming that you state it doesn't take fuel to "counter engine braking", you're wrong. Engine braking is just that, using the engine as a brake. If no fuel is added, pumping losses will make it act as a (relatively weak) brake. If you want to coast without braking, with the clutch engaged, you will need to inject fuel to counter the pumping losses of the engine, losses which are higher if the engine is not idle. Losses in the power train add to the braking effect, which must further be counted with additional fuel. If you're going downhill with clutch disengaged and maintaining your speed, engaging the clutch will very likely increase your fuel consumption for the same task (maintaining speed). Automatics have a coasting gear (usually the last or only overdrive gear) for this very reason. (Sorry, I had to do that.) Side notes: 1. Coasting is not braking. People seem to be mixing "coasting to a stop" and "coasting" in general. Engine braking, when your intention is to reduce your speed, is more efficient than friction brakes, but coasting does not mean that you intend to reduce your speed. 2. Who drives a car with a 2-stroke petrol engine? :)
  • hanoz 10y ago
    The fact that you use extra fuel when coasting isn't enough to make it less efficient, you'd have to be using less extra fuel than the extra fuel you would otherwise have to use to replace the benefit you extract from your potential energy by coasting.
  • J-dawg 10y ago
    Some ECUs also cut fuel on braking. I remember reading a review of a car where the reviewer (obviously an "enthusiastic" driver) was disappointed that he couldn't do left-foot braking because it kept cutting the fuel. (Left-foot braking is a technique where you keep the power down in a bend but dab the brake to adjust the fore-aft balance of the car. It is common in rallying - there is some awesome footage somewhere on YouTube of the driver's feet in an 80s rally car. His feet move so fast it looks like he is dancing on the pedals.)
    • pryelluw 10y ago
      This is one of my pet peeves. Sometimes I use left foot braking to transfer chassis weight from one side to tve other. Having a throttle cut off sucks. I won't buy a car with it anymore.
      • ohyoutravel 10y ago
        I can't think of any situation outside of a race course where this would be a necessary maneuver. If you're going so fast on public roads where you need to left foot brake to transfer weight to avoid skidding off the road, it sounds like something is wrong. Where are you doing this enough where having a car with throttle cut off is enough of an annoyance where you won't buy a car with it anymore? For reference, I got pretty good at this technique while driving autox, which for those unfamiliar is basically a race course around cones in a giant stadium parking lot, one car at a time. But in my 10 years of driving on public roads, I've never felt the need to use it even once.
        • pryelluw 10y ago
          Different driving styles. I focus on safe driving. Being able to transfer weight around bad bumpy roads helps me drive safe. Roads are not great in Puerto Rico and I drive a FWD car. Edit: Downvotes? For driving safely on shitty roads? I hope people don't assume Im being a jackass driver just because I use my left foot.
          • 10y ago
  • TeMPOraL 10y ago
    Oh so that's what was happening... Two days ago my friend and I were driving a rented car and we've noticed that when coasting, the computer said the fuel consumption is 0, but when the friend disengaged the clutch, the fuel consumption actually went up (!) to some value. We've been wondering what's going on, and we actually suspected a software bug in the fuel consumption indicator. Thanks!
    • po 10y ago
      Not only that but staying in gear when going down hills or slowing down will reduce the wear on your breaks by using the inertia of the engine to slow the car instead of break pad friction/heat.
      • irishcoffee 10y ago
        I've always wondered about this. In my head, using the engine/transmission to "brake" when in gear going down hills or slowing down seems like a bad idea. Something is getting stressed in order to slow the car down. Having to replace my brakes makes sense, they get worn out, and its relatively inexpensive (especially if I do it myself). I have always been wary of staying in gear to brake. And, as it so happens, my manual has 130k miles on it, stock engine, transmission, and clutch. Never had any problems. I would love to understand the science behind why using the engine/transmission/clutch to brake is better for the car than just using the brakes.
        • JoeAltmaier 10y ago
          Just what the previous commentor said: saves on wear and tear on the brake pads. As for wear on the engine: the engine is actually designed to mesh with the transmission. Engine braking is not much different than accelerating.
          • at-fates-hands 10y ago
            >> Just what the previous commentor said: saves on wear and tear on the brake pads. When I was learning how to drive a manual, my Dad was a huge fan of them and all of our family cars were manual. I was taught that it makes a huge difference when you're driving in the mountains to use engine braking instead of your brakes. He also explained the difference in braking in traffic and using engine braking to do so and how it supposedly saves on gas as well - something about idling the engine versus continuing to have the engine running? I'd be interested to hear from people who regularly drive in the mountains and the advantages/disadvantages of engine braking versus using your brakes and if that was some myth or actually real.
  • odonnellryan 10y ago
    > where staying in gear allows the computer to completely cut fuel flow I don't think you need a computer for this: it's called engine braking. https://en.wikipedia.org/wiki/Engine_braking https://en.wikipedia.org/wiki/Engine_braking
  • rsync 10y ago
    "In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because it takes some fuel to keep the engine idling, where staying in gear allows the computer to completely cut fuel flow, as long as the forward motion is keeping the engine turning." This is very interesting - thank you. I shift cars into neutral while going down hills because I strongly dislike the subtle lurching of the hill descent control ... I never thought I was saving any appreciable fuel. However, what does happen is that (provided you don't mind speeding) you exit the bottom of the hill with much more stored energy, allowing you to coast (in or out of gear) much further before it is necessary to apply the throttle again. So it's interesting that the direct effect of coasting in neutral does not save fuel - however I still think the net effect (again, provided you don't mind speeding when exiting a hill) is fuel-saving ...
    • rhinoceraptor 10y ago
      It's actually illegal in many states to coast downhill in neutral.
    • nawitus 10y ago
      Coasting down a hill in neutral definitely saves fuel compared to coasting in gear if you can utilize the kinetic energy. If you're coasting down a hill and then need to stop quickly it's better to coast in gear.
  • gens 10y ago
    There is nothing preventing cars with manual clutches from doing that. People going down hills with manual transmission also keep the car in gear (they teach you that). And it doesn't matter as it is not much fuel compared to the fuel used when accelerating, where automatic transmissions are less efficient. If that is the only reason for automatic transmission to be more efficient, then i doubt it is.
    • user5994461 10y ago
      > People going down hills with manual transmission also keep the car in gear (they teach you that). It's taught for safety reasons. You have more controls of the car with the motor attached.
  • mmagin 10y ago
    Yep. Where modern in some cases includes cars from 26 years ago. (My 1990 Chevy Corsica - inline 4, with a single throttle-body injector did this.)
  • astrostl 10y ago
    Well, TIL on that, so thx! Still going to pop into neutral w/o engaging the clutch and coast a ton, as I've always done, though :D
• burfog 10y ago
  Lock-up torque converters don't waste power. They started to become popular in the 1970s, and are pretty much universal now.
  • vmarsy 10y ago
    You're right, however those only work at higher speeds, hence the "especially with city driving" :) From wikipedia : A Lock-up clutch is used in some automatic transmissions for motor vehicles. Above a certain speed (usually 60 km/h) it locks the torque converter to minimise power loss and improve fuel efficiency.
    • Neil44 10y ago
      Negative, current generation transmissions lock up from first gear.
    • yason 10y ago
      It's typical to omit locking while the transmission is cold, some transmissions may not lock the first gear, or may not lock at lower revs, or at slower speeds, but generally the lock is there and it's just software that controls locking. You can rather safely assume that on most modern cars and in general conditions the torque converter will lock starting from the first gear.
  • 3poundkilograms 10y ago
    The lock-up only locks up in very limited conditions, especially older ones. There's basically a few specific speeds with a 10km/h margin that locks up.
    • pkolaczk 10y ago
      My Mazda CX-5 locks up in all conditions, even if it is cold and in the first gear. And it has better EPA than the same model with MT.
      • 3poundkilograms 10y ago
• homero 10y ago
  My dsg has 2 real clutches
• Neil44 10y ago
  Those things have not been true of automatics for the last two or three generations. They have clutches inside the torque converter, they can block shift. But it is only the latest generation of gearboxes that return better economy figures than manuals. Until very very recently auto boxes were indeed much worse for economy.
  • jfitbsidbfb 10y ago
    The TC only locks under light load on the highway, and isn't 100% effective there because the hydraulic fluid still produces a lot of drag.
    • Neil44 10y ago
      That's not true any more, for recent transmissions. They can lock from first gear. E.g. ZF 8 speed, Mercedes 5,7,9 speeds.
• 3poundkilograms 10y ago
  Well, engine braking to the red light would have been more efficient. :) But indeed, automatic cars are less efficient. They do not know what you will do next, so they often make erroneous gear shifts that waste fuel and time. They don't know when to coast or when to engine brake (For older transmissions, the overdrive gear had permanent coasting, and could not engine brake at all), often have lossy pumps, etc. Dual clutch transmission are awesome, though.
  • speeder 10y ago
    I am from Brazil, here the population hates "hydramatic" cars as they call automatic cars. I saw one accident that the cause was lack of transmission knowing the future: road crossing, bad road signs didn't made clear who had priority, so both drivers went forward. Both cars tried then to stop, but the automatic car instead had just shifted up, then as the driver tried to stop, it lurched forward and hit the other car, then it spun and crashed on the wall of a school. The automatic car was a sports car from the 70s though, so I guess the transmission was even more dangerous.
    • 3poundkilograms 10y ago
      Automatic transmissions back then were quite the hydraulic maze, as they operated entirely on hydraulics. These days, they're electronically controlled. I don't think the failure rate has changed. In a manual, you can of course clutch out as long as your clutch cable (or hose for hydraulic clutched cars) doesn't fail, which is not an option you have for automatics. These kinds of failures do occur today, though. See the famous Toyota "unintentional acceleration" scandal, where a software error read "0% throttle" as "100% THROTTLE GO GO GO GO", causing death in a few cases. There's also a case of a person that was stuck in a car that could not stop, driving at constantly increasing speed on the highways of France, IIRC. He couldn't turn the car off or get it out of gear. You can disengage a manual if you're quick with an intact clutch cable, but you're a bit lost in an automatic, especially if it's too powerful to hold on the brake. Up here, manuals are normal (with automatics slowly getting more and more popular), mostly because everyone knows just fine how to drive them, and automatics are much more expensive. I, personally, can't decide which I prefer. I'd much rather drive fully manual than a "retrofitted" automatic like the one in a Lupo 3L, and I still have to try a dual clutch transmission, for which I might end up having similar opinions. They always seem to change wrong, the changes are rough and slow, and the clutch engagement is very binary, leading to odd engage/disengage. I like conventional automatics in cars that haul heavy loads (pushing a 4 metric tonne trailer over a curb in reverse at low speeds without the possibility of gaining some speed first make you worry a lot about your clutch in a manual), and our busses and trucks are generally fully automatic, with more gears than you would bother counting.
• computator 10y ago
  > because of Physics, an automatic transmission will always be less efficient You making that claim based on how automatic transmissions currently work, right? And not on every way they could work. Purely from logic, your claim has to be false: Imagine a humanoid robot sitting in your driver's seat, driving your manual transmission car, its robot arm doing the shifting exactly as a human would. Your manual car has been turned into an automatic and it is not less efficient. I suppose you could argue that the robot, being part of the car, has an energy requirement. I would then argue that we could shrink the humanoid robot down to the size of chip with sensor inputs, and replace the robot arm and gear shift with a solenoid or whatever, thereby saving considerable energy. By this thought experiment alone, I don't see how anyone can make a statement about automatics always being less efficient.
  • fuj 10y ago
    How the hell does this make any sense?
  • caf 10y ago
    "automatic transmission" is usually taken to specifically mean a torque-converter based hydraulic transmission, rather than a general class of self-shifting transmissions. I'm sure that's the sense in which the parent comment to yours meant it.
  • IshKebab 10y ago
    You're just repeating what he said. A dual clutch automatic can be more efficient, but the more common torque converter type won't be.
  • klagermkii 10y ago
    What you describe already exists but it doesn't get called "automatic". The car companies don't call it that, the drivers don't call it that. It'll be called DCT or the older sequential manual gearbox.
  • Aaargh20318 10y ago
    > Purely from logic, your claim has to be false: Imagine a humanoid robot sitting in your driver's seat, driving your manual transmission car, its robot arm doing the shifting exactly as a human would. Your manual car has been turned into an automatic and it is not less efficient. You're forgetting a very important factor: the automatic transmission can only respond to the drivers actions. Unlike the driver it has no knowledge of future plans. It will happily shift up 2 seconds before I hit the brakes because it has no idea I'm planning to take a left turn in 20 meters.
• mcv 10y ago
  Maybe that's true for old fashioned cars, but with the move to hybrids and electrics, all of this becomes irrelevant.
• henrikschroder 10y ago
  In another subthread to this I posted numbers for various models of BMW, Audi, and Mercedes. I could find one Mercedes model where they report worse fuel economy and slower 0-100km/h acceleration time for the version with an automatic gearbox, in all the other models I looked at, the manufacturers rate the automatics as more efficient and having better acceleration than the manuals. These are of course high-end current model year vehicles, if the tech they have hasn't already trickled down into cheaper cars, it will do so very shortly, making my statement even more broadly true.
  • Annatar 10y ago
    Please bear in mind that manufacturers currently self-police and that they test under artificial conditions. Hence, there have already been several scandals with Honda, Ford, Hyundai and Volkswagen either manipulating, or falsifying results; not even the onboard computer can be fully trusted: the only precise measure is noting how many liters were put in and how many kilometers were driven on that quantity, and that only on the spec tire size and with the assumption that the odometer is correct.
• andrewwhartion 10y ago
  > It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency, hence the reason to switch from 3rd to 5th gear directly. I don't mean to be pedantic, but I think your logic is backwards here, ie. It's well known that 5th (top) gear is the most fuel efficient, hence the reason to accelerate to it rather quickly (the optimal rate would vary between cars), I think would be more reasonable. I don't think the cruise speed factors into it, and neither does switching from 3rd to 5th, unless it's part of the strategy to get to 5th faster. But then again, I never realised that accelerating harder to get to 5th faster is actually more fuel efficient [1], so thanks for the heads up! [1] http://www.popularmechanics.com/cars/a6827/6-driving-tactics-to-save-gas-this-weekend/ http://www.popularmechanics.com/cars/a6827/6-driving-tactics...
  • torrent-of-ions 10y ago
    Low gear is best for acceleration and overall control, high gear best for cruising. So accelerate in 3rd (or 2nd if your car is fun) to cruising speed, put straight in 5th/6th and activate cruise control. It's a waste of energy and clutch/gearbox actuation to use the in between gears and there's never anything wrong with being in a low gear at any point unless your engine hits the red line. On most cars fourth gear is really for cruising at a lower speed than is comfortable (or safe) in fifth. You can also very easily downshift sequentially through the gearbox. Downshifting through several gears takes much more practice to do properly and isn't very useful on the road.
    • fnj 10y ago
      I learned to tease my 4-speed automatic TDI into shifting from 2nd directly into 4th, by accelerating strongly to 40-50 and then abruptly lifting throttle.
• mbrookes 10y ago
  Coasting in neutral or with the clutch disengaged is both more dangerous and less efficient then engine braking. In the UK you will fail your driving test if you coast, and in some countries / states it is illegal - with good reason. Just google it - not a single article supporting your argument.
  • 72deluxe 10y ago
    But if you're coasting down a hill and not braking, your engine is idling and you're making use of gravity. The engine will be turning over at ~1000rpm instead of 2000rpm in my diesel. Understandably it is dangerous as you are not in control of the transmission and therefore the wheels, hence the failure in the UK. But I got through some incredibly scarce times by coasting a bit on the commute to work... With automatics you would need to brake down the hill to stop the car "running away" as it wouldn't engine brake automatically to compensate for gravity's effect.
    • userbinator 10y ago
      With automatics you would need to brake down the hill to stop the car "running away" as it wouldn't engine brake automatically to compensate for gravity's effect. Most automatic transmissions (or at least the ones I've driven, which arguably is not more recent than late 90s) have low ranges where there is engine braking. That's the exact use case for them. Automatic doesn't (or didn't) mean that you must never shift manually. Automatic transmissions in large vehicles like trucks and buses usually don't have freewheels either, so there is always engine braking.
    • Steuard 10y ago
      Every automatic car I've ever seen has had the ability to downshift to engage engine braking. (Even the CVT in cars like the Prius has an engine braking mode.) I've seen advocates of manual transmissions cite a lack of engine braking as a reason to switch in multiple conversations, so I've gotten the impression that a lot of people just aren't taught how to drive an automatic properly. (Do they not talk about this in driver's ed? I don't even remember anymore.)
• pkolaczk 10y ago
  "Do you have data to back this up?" Official data (source: https://www.mazdausa.com/siteassets/pdf/features--specs/2016/mazda-6/2016_mazda6_features_specs.pdf https://www.mazdausa.com/siteassets/pdf/features--specs/2016...) * Mazda 6 2.5L SkyActiv-G MT: 25/37 MPG * Mazda 6 2.5L SkyActiv-G AT: 26/38 MPG (28/40 MPG in Grand Touring model, not available in MT) As for the torque converter inefficiency - that is true only for some ancient transmissions. A modern torque converter will waste some energy only during shifting the gears. After synchronization, the torque converter is locked and the connection between the engine and the wheels is mechanical, just as in MT. "It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency" And now - any data to back this up? I don't have hard data, but I have some hard theory. Modern engines have variable valve timing technology which allow them to work in a much more efficient thermodynamic cycle when under light load and low RPMs. When you're not using the full torque of the engine it works in something like an Atkinson or Miller cycle, with high compression ratio and much higher thermodynamic efficiency. A bad thing about Atkinson-like operation (achieved by keeping the inlet valve open longer than normally) is that it reduces the effective capacity and hence power of the engine. So if you press the accelerator pedal fully, the ECU switches the engine to a classical Otto cycle, which is less fuel efficient but gives more power. Hence, if you accelerate fast, your mileage will be worse, not better. Another issue is that keeping high RPM at low gear (possible with turbocharger running on full pressure) to accelerate fast makes it harder to control burning all the fuel efficiently because there is less time to do that in each cycle. And also the temperature of the cylinders goes quickly up which makes it harder to avoid detonation. So in order to not destroy the engine, the ECU will adapt the injection / spark / valve timings and lambda coefficient to avoid explosive burning of the fuel by decreasing the effective compression-ratio and increasing lambda to decrease the temperature. Lower compression-ratio and higher lambda means lower fuel efficiency. That's why when I "eco drive", the ECU keeps the gear extremely high, so that the engine works typically in a range 1000-1500 RPM. Now this is quite hard to achieve in practice when driving an MT car, because whenever you want to accelerate faster, you'd have to gear-down by 2 or 3. That's why ATs are better for fuel efficiency. BTW: I do own 2 cars, one with MT, one with classical hydrokinetic AT, so I have a comparison. I'd really not be able to drive my MT car using such high gears as my AT car can. And also I can't switch gears so fast as an AT can do, even though my AT is not the fastest among all ATs.
• mrfusion 10y ago
  I think modern torque converters actually lock together once the speeds match.
• vgrafe 10y ago
  Disengaging the clutch is not recommended, since it modifies the balance of the car. When you are accelerating, the rear wheels are supposed to be under more pressure, then have better grip. The opposite when using the brakes. Not having that balance (for which the car is designed/tested) can lead to dangerous situations in certain conditions. Also, the gain in gas consumption is very debatable.
• Yaggo 10y ago
  > Do you have data to back this up? I think this is widely known fact for modern cars. Automatics (may) have more gears & can consistently shift more optimally than average human driver, making up the loss of "wet" torque converter.
• ecopoesis 10y ago
  Automatics and manuals have basically the same efficiency. All numbers from fueleconomy.gov 2016 Ford Fiesta FWD 4 cyl, 1.6 L - Auto: 27/36, Manual: 27/35 2016 Ford Focus FWD FFV 4 cyl, 2.0 L - Auto: 26/37, Manual: 26/36 2016 Volkswagen Golf 4 cyl, 1.8 L - Auto: 25/35, Manual: 25/36
  • problems 10y ago
    These can be pretty inaccurate in my experience. It's annecdata, but my brother's Corolla is rated at 6.7 L/100km on highway for the manual, but he had no problem getting 5.6 L/100km in reality. 16% less than claimed on a 10 hour drive. I think they may be vastly underestimating manual drivers ability to optimize this. EDIT: Actually on the US site the Auto is rated for 6.9 L/100km, 7.1 L/100km on the manual. The Canadian NRCAN rated it 6.7 L/100km.
    • jdhawk 10y ago
      42 mpg, 50.4mpg
    • luma 10y ago
      Everyone feels like they are a better-than-average driver. The EPA has a standard testing regime, and modern automatic transmissions nearly always score at parity or slightly better than the manual option mounted to the same vehicle. Interestingly, this is also true for things like 1/4 mile times. "Muscle car" guys hate this, but if you look at the numbers, a modern automatic transmission is both more efficient and more performant than the manual option.
      • deleted 10y ago
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• wil421 10y ago
  That is absolutely untrue. Go look at EPA estimates for the same cars in Auto/Manual. Besides, most manuals are 5 or 6 speed. I was reading about new ZF transmissions and they stated a 14% fuel efficiency gain over a standard 5 speed
  • fnj 10y ago
    What is your point? Lots of automatics are 6-8 speeds now.
    • wil421 10y ago
      My point is automatics have better fuel consumption than manuals. Especially the newer 8 speeds.
• problems 10y ago
  Even many of the CVTs have fake gears because people didn't like the sound and feeling of a straight up linear CVT, so they might be more comparable to automatics.
• ohstopitu 10y ago
  I can give examples of cases where it's NOT true. Subaru BRZ/WRX manuals have better Milage EPA ratings than their automatic counterparts.
  • CWuestefeld 10y ago
    Having spent 12 years with a WRX, and moved a couple years to a Hyundai Genesis Coupe; while my wife still drives a Subaru Forester... Subarus are a bad example here. They've got great engines and other factors, but their automatic transmissions really suck. Is there anywhere else where you can get what's supposed to be their high-performance car (my WRX was the year before STI came out) or the performance variant of another (my wife's Forester is an XT), where that automatic transmission is just a 4-speed? And subjectively, it's a rather balky one at that, with very slow shifts. For comparison, my Genesis Coupe's auto tranny is an 8-speed, which maximizes the time that the engine is at its most efficient. It's also got smooth lightning-fast shifts in comparison to either Subaru.
    • benmcnelly 10y ago
      I have a manual 2015 Subaru WRX, and I can get way better MPG number than listed, if I want to. I don't, but I can.
  • Grishnakh 10y ago
    I call bullshit. Last time I looked up the BRZ, the automatic was far better than the manual, something like 36mpg vs 30mpg (highway). Most likely, the manual has much lower gearing than the automatic.
• Unklejoe 10y ago
  I don’t have data on hand, but there are some issues with your claims. They're accurate when comparing a manual transmission to an automatic from the 70's, but newer automatics have essentially closed any efficiency gap. 1. On most automatic transmissions (probably anything newer than the early 90's), the torque converter has clutches inside of it which allow the pump to be directly locked to the turbine. This makes it behave basically the same as a clutch in a manual transmission - no constant slipping. The converter will usually “lock up” once you’ve reached a cruising speed. So, it is less efficient during acceleration, but once the converter locks up, it’s much more comparable to a regular manual transmission. The only thing that still hurts the automatic at that point is the fact that there’s a hydraulic pump which must be driven at all times to maintain pressure to keep the various clutches clamped. So yes, this is where city driving can show some efficiency differences. However, ... 2. On newer automatic transmissions (i.e., transmissions from probably around 2006 and up, such as the ZF 6HPxx series which is found on many Fords, BMWs, and some other brands), the torque converter actually behaves even more like the clutch in a manual transmission in the sense that it’s far more “aggressive” regarding when it locks up. For example, my 2011 BMW will usually lock the torque converter as soon as you take off in first gear (it even sort of feels like when you’re taking off in a car with a manual transmission). Then, it remains locked the entire time after that (even during shifts and wide-open-throttle conditions). 3. Newer automatic transmissions (such as the ZF mentioned above) do in fact have the ability to jump to any gear. I believe this is one of the main things that separate transmissions like the ZF 6HPxx from older style electronic automatics like the 4L80E which use a sprag system. The newer transmissions have a clutch pack for every gear which allows them to jump to any gear rather than following the sequence. 4.I believe that many CVT transmissions still use a torque converter anyway.
• Bartweiss 10y ago
  This was my general impression - especially the early shift issue. I learned to shift manuals ~500 rpm higher than where most automatics seem to, a decision that seems made to produce the smoothest possible automatic acceleration at a cost to efficiency. Notably, this feels way less pronounced with faster, sporty automatics, which are willing to give you a feel of acceleration and shift more like manual drivers.
• wired_devil 10y ago
  Here, we are instructed to use the clutch only for switching gears. At least in your driver's licence test.