By on August 25, 2008

Go ahead.  You know you want to."What is more fuel efficient for automatic cars, accelerating quickly or slowly?  And what is generally peak torque for typical passanger cars and trucks? For manuals, quick and smooth acceleration is most efficient.  I would expect that the same is true for modern automatics, but don’t know for sure.  With my own automatic trans car, I notice no difference between the two, maybe a slight improvement with faster acceleration.  In the past, a study was done comparing slow braking and acceleration with fast braking and acceleration, but they didn’t investigate fast acceleration with light braking. I’ve been accelerating quickly (but keeping RPM below 3500 where I suspect efficiency drops off) and adjusting my speed mostly with the accelerator (easing off earlier to provide a little extra safe following distance and keeping near the speedlimit to time traffic lights).  I get pretty good gas mileage, between 26 and 30 MPG (mostly sub-urban highway driving during rush hour). Before I started accelerating more quickly I got about 26-27 pretty consistantly. Is the efficiency of slow acceleration just a myth?"

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46 Comments on “Ask the Best and Brightest: “Is the efficiency of slow acceleration just a myth?”...”


  • avatar
    Morea

    Isn’t wide open throttle where pumping losses are least? Shouldn’t you floor it until you get to the torque maximum and then shift (a manual) transmission?

    With automatics being so smart these days even the engineers who designed the car may not be able to answer the question.

  • avatar
    Mirko Reinhardt

    If I drive economically, I change gears at about 1800 rpm, but use the accelerator pedal “digitally” (on/off). That’s what gives me the best results.
    I have a manual. Maybe slushboxes should have a “treehugger” mode for that kind of driving.

  • avatar
    SupaMan

    Wow….I do the same thing as well. Accelerate quickly and keep the revs below 3000rpm while lifting off the gas between shifts (I miss the MT that much). On the highway, the revs stay between 2500 and 3000rpm. Above that and not only do I notice efficiency drops but the engine starts to sound like a lawnmower.

    But hey…33mpg average.

    And it’s a *sigh* automatic.

  • avatar
    dougw

    BMW did this research in depth when developing the 528E. I remember reading an extensive article about it at the time.

    Their conclusion was that relatively brisk acceleration up to the intended speed was the most effecient. The rather obvious reason being getting to higher gears quickly drops the fuel consumption as the travel distance/revs improves. Also, pumping losses were more severe at light loads. And as I recall it was true for both manual and automatic.

    If you have a fuel economy readout you can observe this yourself. It is interesting to watch the instantaneous mileage numbers change with each shift.

    Bottom line, 1st, 2nd and 3rd gears really suck. The longer you hang around in them the more fuel you are using.

    But obviously this assumes you are running up to steady state cruising. If there is another stoplight ahead the equation gets far more complex.

  • avatar
    KixStart

    It is probably slightly more efficient to change gears up quickly and keep the pedal on the floor but you’re better off investing your attention in managing your speed so you don’t have to stop at the next intersection and have the brakes turn all your beautiful kinetic energy into waste heat.

  • avatar
    Airhen

    I notice a lot of people these days doing snow take off’s, which I completely agree that slow take off’s do not improve the mpg. With my Jeep that sucks gas, I want to get it to where the over-drive kicks in and runs the best in, but as well not too fast that my Jeep’s wind resistance is higher. Speeds between 50 and 65 mph with the over-drive on is where I can be the lightest on the gas peddle.

  • avatar
    Ralph SS

    Well, by cracky, I remember a little thing from the past called a carburator. Mostly configured in 2 barrel and 4 barrel designs. When the first oil crisis hit most cars on the road were carburated. The theory at the time was that if you kept your foot out of it you would keep the secondaries and the injector pumps at bay one would get much better mileage.

    Then fuel injection and computerized engine management came to play and all that went out the window.

    So, what was once fact became myth.

  • avatar
    seoultrain

    Can anyone explain WOT economy driving? I don’t really care about mileage since I don’t drive enough to warrant changing my driving style at the expense of fun, but it would be cool if I could floor it AND save gas. Do you just short shift into 2nd, floor it, and continue to shift at low rpms?

  • avatar
    blautens

    Bah! With my 4 speed auto and a 4.10 rear end, it matters not to me…cruising speed is no more efficient than accelerating.

  • avatar
    WaftableTorque

    I drive fairly fuel efficiently, so I find environmental factors affect my fuel economy more than acceleration habits.
    I usually lose 1 mpg when temperatures drop below 28 degrees Celsius, and another 1 mpg every 15 degrees or so.
    I also lose 4 mpg going from all-seasons to studded winter tires, the rolling resistance being so high.
    Using the air conditioner I don’t notice any mileage difference, probably because when it’s hotter, my cars get better mileage to make up for it.

  • avatar
    konaforever

    Any excuse for flooring it sounds good to me.

  • avatar
    NickR

    blautens ‘Bah! With my 4 speed auto and a 4.10 rear end, it matters not to me…cruising speed is no more efficient than accelerating.’

    What are you driving, a Six Pack Superbee?

  • avatar
    Gardiner Westbound

    Experts I consulted in my car and truck fleet management days agreed driving in the highest possible gear best improves fuel efficiency. Modern automatic transmissions have eliminated concerns about lugging the engine.

    Toyota’s transmission hesitation issue is widely attributed to programming that upshifts to the highest gear early and holds it as long as possible. Toyota won’t risk the EPA’s wrath by reprogramming the transmissions. Much better to stonewall unhappy customers!

  • avatar
    barberoux

    Wouldn’t it have to due with the efficiency of the torque converter in an automatic? Automatics all have built-in inefficiencies until they lock-up. Does accelerating hard change the efficiency of the torque converter? I think the efficiency would go down, i.e. there would be more loss of efficiency where engine torque is not converted to wheel torque. Also when accelerating hard my cars usually heat up more therefore causing more energy loss through heat, again inefficient. Minimal braking would help since you are not bleeding off energy into heat but assuming that the braking losses are constant I think hard acceleration would not be more efficient, in an automatic.

  • avatar
    bunkie

    “What are you driving, a Six Pack Superbee?”

    I don’t remember the Torqueflight A727 having four gears…

  • avatar
    Morea

    However, always driving at very low engine speeds will build up carbon desposits on the exhaust values and lower engine efficiency (especially when using inexpensive gasoline without additives).

  • avatar
    virages

    Ok, maybe I don’t know enough about ICE engineering and automobile mechanics, but I do know a bit of physics. To accelerate an objet faster, you have to use more energy. It will always require less energy and less gas accelerating slower. It is simple physics.

    However there are some conditions where this may not apply, for example holding low gears when they are not needed. Sure you may not exceed say 35mph and accelerate slowly, but if you stay in second gear, you will use more gas.

    In my car, if I am not trying to gun it, I can comfortably be in 5th gear (Manual Tranny) by the time the car is at 25mph, and slowly accelerate to the desired speed. This is pretty much the most efficient way to accelerate with the minimal usage of gas. With $8 per gallon gas here, I’ve thought about it.

    But when it comes down to it, acceleration and deceleration are going to be the part of driving that costs you the most in gas. Keeping a car moving doesn’t use much energy (with the exception of drag at higher speeds). So some one who does drag race starts, but drives on the highway, will get better milage than the snowflake starter that drives in the city… get it?

  • avatar
    menno

    I use one method on my wife’s Sonata (2007 4 cyl automatic) when possible, and it seems to work; that is, gentle throttle, allowing the transmission to shift at 2000 rpm or slightly above, and allowing it to get up to 45 mph reasonably quickly so the top gear AND torque convertor lock-up can come into play. When needed (heavy traffic), I use maximum throttle and then back off at the right speed.

    With the Prius (2008) hybrid, I generally accelerate more briskly because I can SEE how the MPG (literally) sucks by creeping up to speed.
    The alternative method (in TOWN) is to ease up to speed if the battery is fairly full and let the electric mode work (barring holding up traffic – if there is a lot of traffic, I don’t do this). Usually, when getting up to 45 or 55 mph (i.e. above the 40 mph where electric only can work on the Prius), I fairly zip up to speed, then back off and let it cruise. MPG goes from 15-20 to 45-70. The less time spent at 15-20 mpg, the better. As necessary (again, heavy traffic) I’ll go WOT and then back off at the speed limit.

    I also recall the BMW studies widely disseminated re: the fuel-efficient 528e. (e for “eco” or “economy” presumably).

  • avatar
    mdf

    dougw: But obviously this assumes you are running up to steady state cruising.

    If the acceleration is to a steady state, who cares if it is lead-foot-floor-it or and egg-shell-ooze-it? The gas you burn on the ramp is going to be a blip in the face of the long-haul.

    If there is another stoplight ahead the equation gets far more complex.

    In fact, the variance in fuel economy I observe appears to be due completely to external factors like other traffic, red lights, and so on. There are days when I pull off 3.6L burned, and other days when it is over 5L burned .. on exactly the same route.

  • avatar
    mdf

    virages: It will always require less energy and less gas accelerating slower. It is simple physics.

    The simple physics is that energy is energy, and it is conserved. Slow acceleration or fast acceleration, you’ll need the same amount to reach some fixed speed. More due to conversion inefficiencies.

  • avatar
    SupaMan

    konaforever :

    Any excuse for flooring it sounds good to me.

    Just make sure your engine sounds good doing it. Otherwise, it’s just agricultural noise.

  • avatar
    steronz

    I suspect that the nature of most people’s commutes renders the answer basically meaningless. Assuming 100% efficient engines and drivetrains, and driving in a vacuum, there’s no difference between a 5 minute trip where you spend 10 seconds getting up to 60, 4 minutes and 40 seconds driving at 60, and 10 seconds coming to a halt, versus spending 2 minutes slowly crawling up to 60, 1 minute at 60, and 2 minutes coasting to a stop. The same amount of work has been done, since the mass of the car and the distance travelled are all that matter.

    However, add in air resistance, and then it seems to make sense not fighting all that air at 60mph. If going from A to B at 35mph is more efficient than doing it at 60mph because of air resistance, then spending more time at 35 and less time at 60 (i.e., the slow acceleration/slow braking method) would make sense. However, over the course of a 20 minute highway commute, this is going to be irrelevant. Furthermore, spending 2 minutes to get to 60 is an exaggeration… in reality, we’re probably talking about a difference between 12 seconds for “brisk” acceleration and 25 seconds for “crawling to 60″ mode. Those 12 to 13 seconds aren’t going to matter much, even if you have to do it several times throughout the course of a commute.

    I suspect that to really notice a difference, you’d have to set something up wherein a car goes from 0 to 60 to 0 every mile for a few dozen miles. Then I could see the slower acceleration paying off, simply because you’d spend less total time at speeds where air resistance is killing economy the most. You’d also arrive substantially later than your counterpart.

  • avatar
    chinar

    on the contrary, it does not take more energy to accelerate faster. It takes more power. The net change in vehicle + engine energy from 0 to some final speed is the same.

    IMO, there are three things that need to be taken into account:

    1. efficiency of engine at various rpm, throttle positions : engine efficiency is compromised at very low throttles. thus its best to accelerate “smartly”

    2. torque converter losses: the torque converter clutch will not lock up at very low engine speeds. i believe this is to prevent lugging the engine and also for nvh reasons. but if you accelerate too hard, the lock up will again be delayed to get some torque multiplication effect from the torque converter. thus there is some optimum throttle position that will allow quick lock up and thus max efficiency

    3. energy loss in upshifts: this is an often neglected aspect. if you rev the engine to some rpm and then upshift, you are reducing the kinetic energy of the engine. Thus part of the energy you put into the engine and transmission inertia to speed it up is wasted as heat in the trans during an upshift. thus you dont want to rev too high

    in conclusion, very slow acceleration isnt the most efficient (engine efficiency is low, TC will not lock up)

    hooligan driving is not the best way either (TC will not lock, wasted energy if shift rpms are high)

    thus, the best way is to accelerate “smartly” such that you get the best combination of engine effciency, TC lockup and low shift energy.

    if your final cruising speed is high such that aero drag matters, then it skews the optimum acceleration towards the slower side as covering more distance at the high speed will mean more drag losses

  • avatar
    blautens

    NickR :

    What are you driving, a Six Pack Superbee?

    More cargo space, but sort of…a mildly tuned of these.

  • avatar
    Power6

    @virages: Ok, maybe I don’t know enough about ICE engineering and automobile mechanics, but I do know a bit of physics. To accelerate an objet faster, you have to use more energy. It will always require less energy and less gas accelerating slower. It is simple physics.

    I think what you are missing is the physics of the engine itself. There is a big problem with a gas motor and accelerating slowly, and that is the throttle plate. In order to limit air-flow (and hence power output) the throttle plate must “choke-off” the engine, increasing pumping losses. This is not necessarily the most efficient way to run the motor. Ideally the ICE system would be designed to only take in the perfect amount of air on every stroke with no restriction in the intake tract. (BMW has done this with Valvetronic and eliminated the throttle plate. And many diesel motors already enjoy this advantage of no throttle plate as power in a diesel motor is commonly controled by fuel volume, not air volume.)

    So there is a case for high throttle angles, limiting overall power by keeping RPMs low and short shifting. I have experimented with this myself in my SRT-4, which will happily cruise in 5th at anything 25mph and over. More throttle and short shifting is worth about 1-2mpg in city driving. It’s just a more efficient way to operate a gas motor.

    I imagine an automatic trans makes it more difficult because the throttle angle also controls the shift points so there is no way to independently separate those unless you have one of those manual modes.

  • avatar
    Power6

    However, always driving at very low engine speeds will build up carbon desposits on the exhaust values and lower engine efficiency (especially when using inexpensive gasoline without additives)

    And another generation of the “blow out the carbon” myth ensues…poppycock!!

    Running rich under full throttle (and make no mistake all cars run rich at WOT) is more likely to deposit “carbon” than clean stoichiometric cruising. If you end up with carbon deposits down the road anyways there was little you could do about it and there are better ways to clean it up than driving habits, like proper maintenance.

    To address the other statement there about “inexpensive” gasoline, it’s hard to know what additives are in what brand of gas, and the persistent myth that the high octane gas contains some sort of “better” additives is not likely. Can you point to any empirical evidence of this being true? Anyways if you are worried about gas quality just buy from this list of retailers and rest assured that you are getting good stuff. 6 automakers have blessed the standard, and the requirement for certification stipulates that the cheap stuff passes the same certification as the high-test…

  • avatar
    Morea

    Running rich under full throttle (and make no mistake all cars run rich at WOT) is more likely to deposit “carbon” than clean stoichiometric cruising.

    It’s not a stoichiometry issue it is a temperature issue. Sustained wide open throttle operation will heat up the carbon deposits. Residual hydroxyl radicals will covert them to carbon monoxide and remove the carbon from the exhaust system. The catalytic converter will change the carbon monoxide to carbon dioxide.

    That the whole gas additive issue is a snake pit I’ll agree to that. Cars with “sticking” values seem to clear themselves up by using certain brand name gasolines with additives specifically designed to combat this. That’s been my experience, your mileage may vary.

  • avatar
    OB 50

    How does forced induction affect this whole principle?

    Is it a matter of accelerating briskly, yet staying out of boost, or does that even make a difference?

    I find this question personally interesting since my Mazdaspeed3 isn’t exactly the most fuel efficient thing on the road. I end up driving it around like a grandma a lot of the time, just so I don’t feel like I’m completely throwing fuel economy out the window.

    It would be nice to know that giving it a little extra gas to get up to speed quicker might not be so bad in the long run.

  • avatar
    Lumbergh21

    Normally, I try to accelerate at WOT with low rpms and anticipate red lights, etc. as much as possible to avoid heavy braking. Like others have stated, WOT is the most efficient operating mode for the engine, so as long as I use as high a gear as is safe for the engine, the better the total efficiency of the engine should be. Since I am shiftig “early”, this does not equate to rabbit starts. I don’t see how this could even happen with an automatic, as heavy throttle causes the transmission to kick it down a gear, or two, to provide quick acceleration at the expense of high revs and fuel economy. By the way, this is how I’ve seen them run the vehicles for how far can you go on a gallon/quart/whatever amount of gas type races. Controlled WOT burns followed by coasting.

    As far as a turbocharged engine, like a Mazdaspeed3, staying off the boost is the best way to achieve better gas mileage. The way I look at it, the point of a turbocharged 4 is to provide four cylinder fuel efficiency when it is driven like grandma owns it combined with 6 cylinder power (or better) when you floor it.

  • avatar
    Power6

    It’s not a stoichiometry issue it is a temperature issue. Sustained wide open throttle operation will heat up the carbon deposits. Residual hydroxyl radicals will covert them to carbon monoxide and remove the carbon from the exhaust system. The catalytic converter will change the carbon monoxide to carbon dioxide.

    You’ve out scienced me! I admit I don’t know much about how the carbon gets burned off. But surely a properly running modern fuel injected car burns gas fairly completely and cleanly so carbon deposits don’t build up in the first place? (I guess cold starts could be the issue here)

    How does forced induction affect this whole principle?

    Is it a matter of accelerating briskly, yet staying out of boost, or does that even make a difference?

    I think turbocharging complicates things because you generally want to stay out of it for fuel economy, although normally using high throttle angles only at low RPMs will limit boost anyways. Ideally you could switch to a less agressive “boost map” in your ECU for economy driving where only aggressive stabs or WOT would signal the ECU to order up the boost.

    I was excited in part when I got my Mopar “Stage 2″ turbo upgrade kit for my SRT-4 becuase the Dodge boys engineered in a little thumbwheel to adjust the maximum boost. Really useful for the winter when I run snow tires, I just dial it down to “0″ so even when I get excited I don’t blow away the Blizzaks. I was hoping that this would allow me to run low octane fuel and get better economy, but it turns out the maximum boost is limited in the lower 2 settings, but not the boost “ramp” which is still the aggressive “Stage 2″. I can still get decent fuel economy, I just have to shift really short.

  • avatar

    I remember the same article on BMW’s research as dougw. I think it was in a 1980s R&T.

    Essentially, you want to accelerate the engine at 80% load–not WOT, where the engine will run rich–and shift low, with efficiency plummeting over 2,400 rpm or so. This will vary some from engine to engine, of course.

  • avatar
    cdotson

    The technique for most efficient acceleration will vary, but they all revolve around maximizing brake-specific fuel consumption during acceleration. Since BSFC varies both with RPM and with engine load and is specific to a given engine design you have to know something about your vehicle to gauge your technique.

    Generally you need to maximize the engine load without entering open-loop fuel enrichment and shift at a low enough RPM that piston ring friction becomes a significant internal loss. I have heard between 1200-1500 feet per minute mean piston speed is a good rule of thumb for how high to rev an engine. This all assumes that you will be attaining steady-state cruise at an efficient speed for a time period substantially longer than it takes to accelerate to speed. You still want to coast as long as possible to conserve momentum as coasting deceleration uses less fuel than steady cruise and abruptly stopping.

    As mentioned above I’m not sure how easily this can translate to slushbox automatics since larger throttle openings delay shiftpoints. With a manumatic you might have some better luck but typically the drivetrain controls have a programmed nanny that limits lugging too much for my tastes.

  • avatar

    WOT will provide the most efficient pumping

    just keep the RPM low and you’re gold (I WOT my Civic SI to 3000 rpm, blitzing through all six gears, and get better fuel economy than when I try to baby it but go up to 4000 rpm).

    The problem with WOT is that people bury it and don’t stop at say, 3000 rpm, but go to redline.

  • avatar
    quasimondo

    This seems kinda pointless, like discussing the most effective way to have a passenger car tow a 28-foot yacht.

    If you want more efficiency, buy a more efficient car.

  • avatar
    cdotson

    Driving more efficiently is unfathomably more cost-effective than purchasing a vehicle to save money on gas if your existing vehicle is paid off. If you are able to trade one car payment for an equal one that will end before the car’s lifetime does then you might be OK with buying a more efficient car (and lucky to swing that), but for most of us out there it’s cheaper and easier to drive more efficiently.

    And who would argue that anything American motorists do as drivers constitutes a standard that cannot be improved upon?

  • avatar
    quasimondo

    Who says you have to purchase a new vehicle? Sell your larger vehicle and use the money from the sale to pick up a used Corolla/Elantra/Cavalier/etc. You’ll probably end up making money off of the whole deal unless that vehicle is incredibly old.

  • avatar
    Nemphre

    “Can anyone explain WOT economy driving? I don’t really care about mileage since I don’t drive enough to warrant changing my driving style at the expense of fun, but it would be cool if I could floor it AND save gas. Do you just short shift into 2nd, floor it, and continue to shift at low rpms?”

    That’s right. I use this technique combined with others and get 45mpg in a Corolla. I shift at about 2200 rpm. I’ve heard people say to use 80% because the ECU will richen it up at 100 but I haven’t found this to be the case. For one, it’s very difficult to judge how much 80% throttle is, get to it quickly, and hold it. I tried “80%” and actually got 1mpg less than usual. WOT is simple and more fun.

  • avatar
    Power6

    This seems kinda pointless…Sell your larger vehicle

    This is a circular argument…once one buys that “more efficient car” the question is still valid about how to drive it in the most efficient manner.

    I drive an SRT-4 and I am not about to trade for a Corolla. I run sticky tires with lots of rolling resistance. I get 7MPG on the track. But there is still no reason why I wouldn’t want to drive for the best fuel economy on the street. I am going slow anyways so why not shift a little earlier.

    I guess that doesn’t make sense to some people. To each his own. My priorites with money are not to spend it where it does not offer a decent return of satisfaction. Maybe driving a track day for me is a decent return. Spending the money on gas to get to work is not a bad return either. But ponying up extra gas money because someone else thinks it’s “pointless” to get another 1-2mpg…I’m not buying that just like I am not buying an Elantra or a Cavalier.

  • avatar
    Landcrusher

    To hell with the science. Stomp on it and pay.

    I don’t think the BMW research translates all that well without charts and graphs and more details.

    Certainly, there is some point of diminishing if not negative returns, but I doubt many people drive that slow outside of Florida.

  • avatar
    ctoan

    Some people have mentioned the fuel mixture running rich at WOT, which makes me wonder: doesn’t a rich fuel mixture destroy the catalytic converter? Needing a new one of those will get rid of your fuel savings pretty quick.

  • avatar
    Prado

    When gas prices peaked I tried slow acceleration (keeping the tach under 2k) for a couple of tanks of gas in my 4runner and gained about 2mpg. Yes it works! But driving that way drove me crazy! Experiment over.

  • avatar
    amcadoo

    I recently got 28 mpg in my E92 M3 on a 350 mile trip.

    I accelerate to 100 – 110 then coast to 55 .. repeat. Cruising at 80 I get more like 22-23 mpgs. Same average speed.

    I’ve been able to get 22 in mixed with this driving style. My only concern is premature clutch issues.

  • avatar
    ekaftan

    When you drive around town, I always try to remember that what burns fuel is braking. If you think about it in terms of energy conservation, the only energy wasters (ie energy not doing useful work) are aerodinamic loses, rolling resistance and brakes.
    Aerodinamic you can only make better by driving slower, and around town it does not matter that much.
    Rolling resistance you can improve by using better tires and inflating them properly and also by making sure your car is well mantained.
    So it comes down to brakes. If you drive avoiding the brake pedal, your city fuel efficiency will improve a lot. You’ll also be driving safely.

  • avatar
    Morea

    Power6: But surely a properly running modern fuel injected car burns gas fairly completely and cleanly so carbon deposits don’t build up in the first place?

    Yes this is correct. I would be concerned though with someone shifting at 1800 rpm all the time (as a previous poster suggested). I believe this is a recipe for the build up of deposits on the exhaust valves. In the long run your gas mileage will go down.

  • avatar
    Nemphre

    “Some people have mentioned the fuel mixture running rich at WOT, which makes me wonder: doesn’t a rich fuel mixture destroy the catalytic converter? Needing a new one of those will get rid of your fuel savings pretty quick.”

    Maybe I should mention that my check engine light has been on for a year. I guess it could be the cat. No point in getting it examined though.


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