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The Almighty Buck Transportation Technology

Fuel Efficiency and Slow Driving? 1114

vile8 writes "With the high gas prices and ongoing gas gouging in my hometown many people are trying to find a reasonable way to save gas. One of the things I've noticed is people driving exceptionally slow, 30mph in 45mph zones, etc. So I had to take a quick look and find out if driving slow is helpful in getting better mileage. I know horsepower increases substantially with wind resistance, but with charts like this one from it appears mileage is actually about the same between 27mph and 58mph or so. So I'm curious what all the drivers out there with the cool efficiency computers are getting ... of specific interest would be the hemis with MDS; how do those do with the cylinder shutoff mode at different speeds?" Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?
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Fuel Efficiency and Slow Driving?

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  • Fuel economy (Score:5, Informative)

    by JWSmythe ( 446288 ) * <jwsmythe@jwsmy[ ].com ['the' in gap]> on Sunday October 12, 2008 @06:58PM (#25348607) Homepage Journal

    I spent some time researching this matter after a discussion at work started about it.

    Something that I had observed in my car was that my fuel economy increased as my speed increased.

    At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg. And at 65, i got about 20mpg. This testing was done along I-10 between Jacksonville and Los Angeles. There's lots of room to set the cruise control. A test usually consisted of fueling up, then a hard acceleration to the testing speed and setting the cruise control to handle maintaining the speed for the next 300 to 350 miles. Individual tests were spot checked (repeated somewhere else on the drive).

    In researching this, it wasn't a matter that my car is "faster", stronger, or just plain cooler. It's a function of the drag of the vehicle and the RPMs the engine is turning.

    Most cars make their best fuel economy somewhere between 1800 to 2200 rpm. Ah ha! My car has a 6 speed stick. If I'm in 6th gear it's turning about 2000rpm at 85mph.

    I then compared ground speed to engine speed ratios of other cars, partly selected because they were owned by people in the discussion, or because they were fairly common cars. Depending on the vehicle, it's best cruise speed could be anywhere between 45mph to 90mph.

    • Re:Fuel economy (Score:4, Insightful)

      by Rei ( 128717 ) on Sunday October 12, 2008 @08:11PM (#25349111) Homepage

      I can confirm that in my Saturn, optimal highway fuel efficiency is 55 to 60mph. I've tested this quite extensively. If you follow hypermiling discussions, for most people, their experience is quite similar. If I drive my Saturn at 80mph, I get about 30mpg. If I drive at 55-60mph, I usually get just over 40mpg. On a good trip, if I combine it with shifting into neutral for downhill runs, follow large slow-moving vehicles (no, not tailgating; I always keep a safe distance), and so forth, I've gotten 45mpg out of it. This is repeatable and has been determined over dozens of documented fillups.

      In city, I haven't been able to collect good data about whether my city hypermiling techniques are helping significantly or not because my partner does most of the city driving on the same vehicle, so it messes up my numbers. I don't do the dangerous things like shutting off the engine or doing breakneck turns, but I do accelerate slowly, look way ahead and take hills into account, coast to red lights, time lights, take turns at moderate speeds, and avoid roads with stop signs. Given that I use my brakes only a fraction as much, I *should* be getting significantly better mileage, but unfortunately, I have no way of knowing.

    • TransAms (Score:3, Interesting)

      by solprovider ( 628033 )

      I was going to post almost the same information. I was surprised another car also receives the best fuel economy at 85mph; most cars seem to like less than 60mph. Then I found your post mentioning you have a '00 TransAm WS6. My numbers are from a '99 TransAm and an '02 TransAm WS6; both 6-speed manuals. (I upgraded because they were being discontinued.) Like yours, 85mph is best; over 90mph starts eating fuel, and under 80mph loses at least 2mpg. My WS6 has never beaten 24mpg. The '99 reached 26mpg g

    • Re: (Score:3, Insightful)

      by nmg196 ( 184961 )

      > At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg.

      It's comments like this which us Europeans wonder if there's any point in us trying to be green, when Americans are still driving cars which only do 24mpg. For every one of us in Europe that buys a car which will do 50 or 60mpg, there's always going to be some American buying an tank which only achieves the low twenties.

      Why do I even bother?!

      (yes, I realise European gallons versus US gallons are differnet and I have taken this into accou

  • by Anonymous Coward on Sunday October 12, 2008 @06:58PM (#25348609)

    But I've noticed I get less tired if I walk rather than run :-)

  • by Joce640k ( 829181 ) on Sunday October 12, 2008 @07:07PM (#25348669) Homepage

    Make those fuel consumption displays mandatory.

    Most cars these days know their consumption - it's one of the first things they look at when they connect the laptop to the engine when you go for a service.

    Make the display mandatory, make it large, and put it in a prominent place. It'll do wonders for everybody's fuel consumption.

  • by base2_celtic ( 56328 ) * on Sunday October 12, 2008 @07:08PM (#25348679) Homepage Journal

    An American Road & Track issue from many years ago (and I'm damned if I can recall which one) had a long article on the results of some fuel economy studies conducted by BMW.

    The findings seemed to show that driving style was more important than overall speed.

    The tips, in general, were:

    - Keep your speed constant; fluctuations up and down are bad.
    - Accelerate to your target speed quickly. Spending time slowly accelerating up to it wastes fuel.
    - Be in the highest gear feasible for your engine type and road speed.
    - 75% throttle for acceleration, conditions permitting.
    - Keep your revs low, and change gears often to keep them low. That said, know your torque curve, and use it; if you have a small 4 cylinder, trying to accelerate at 1000 revs is futile.

    • by Lumpy ( 12016 ) on Sunday October 12, 2008 @07:23PM (#25348775) Homepage

      Myth #1

      Accelerate to your speed quickly. This actually wastes gas. It's usually touted by people that really dont know how cars work.

      accelerate in your engine's economy band. this can easily be found by watching your MPG gauge or using a $12.95 Vaccuum gauge attached to your car's vac system.

      Flooring it to your speed wastes gas, you are running rich the entire time putting fuel out your tailpipe. Going to slow wastes fuel as well, accelerate as to what your car's max economy is for that driving situation. problem is most cars are not equipped with the gauges needed to do this. American cars are designed for really stupid drivers, so they remove most of the gauges. too many gauges confuse american drivers.

      75% is inaccurate for most cars. If I was driving a high performance car, 75% throttle is burning tires. In a smart car it's too little as it's power band from a stop is a gradual increase from 45% to 80% as your speed increases so you can keep the engine in it's power band for max economy.

      Basically you have to learn your car. It takes time and efffort to maximize fuel economy. as well as getting rid of retarded driving habits like the morning dragracing from light to light. Accelerate slowly and time the lights to you never have to stop saves more gas than anything else.

      Oh, Car and Driver reviewed those BMW tips, they found that they contradict each other.

      • by tylernt ( 581794 ) on Sunday October 12, 2008 @08:35PM (#25349303)

        Myth #1

        Accelerate to your speed quickly. This actually wastes gas. It's usually touted by people that really dont know how cars work.

        Sounds like you really don't know how cars work, then. Older mechanical fuel injection systems or carburetors CAN get better mileage with full-throttle acceleration (if you keep the RPM down using a manual transmission). The reason is the open throttle lets the engine breathe easier so it's not wasting energy drawing air past a restricted opening. BMW and others have experimented with eliminating the restrictive butterfly to improve economy, and of course one of the reasons diesels enjoy better economy is because they have no throttle butterfly.

        So, yes, you can improve economy by keeping that throttle open and the RPM low -- as long as your computerized fuel injected engine doesn't perform WOT enrichment (or you disable that feature).

      • by base2_celtic ( 56328 ) * on Sunday October 12, 2008 @09:30PM (#25349801) Homepage Journal

        tylernt covers the "full throttle" component of this in another reply to your post.

        I'm an Australian, and the son of an engineer who restores old vehicles for a living. We have British, Italian and Japanese vehicles. We have never owned an American vehicle.

        The BMW data was almost certainly collected for their vehicles, which almost uniformly (at the time of the study) used straight 4 or straight 6 engines. 75% throttle would have been a rough figure arrived at for their own machines, I would imagine.

        It is patently obvious that applying more throttle increases the amount of fuel used per second. However, the amount of fuel used is not a direct 1:1 to your acceleration.

        The trick here is not that you use less fuel to reach your desired speed by accelerating harder. That's nonsense, and an incorrect understanding of the problem. Accelerating harder may well use more fuel to reach your desired speed. The trick is in how much time your spend at you desired speed, not accelerating.

        If you do the calculus on this, you'll note that with the rapid acceleration model, you spend a far greater time at your desired target speed over the course of your journey. While at that speed, you are not accelerating. You will use more fuel accelerating quickly than accelerating slowly, per unit of time. However, your overall time spent not accelerating but simply maintaining speed more than compensates.

  • by kbrasee ( 1379057 ) on Sunday October 12, 2008 @07:10PM (#25348685) Homepage
    Do like everyone else does, drive about 6 inches behind me at 65 mph.
  • by glitch23 ( 557124 ) on Sunday October 12, 2008 @07:10PM (#25348689)

    There are sweet spots for driving which is usually specific to the type of vehicle, the gearing, etc. so, to an extent, I'm sure the faster you go the better MPG you will see. But for my car, Mitsubishi Spyder, they recommend shifting into 6th at about 50mph. So basically my interstate driving is all in the top gear by far. At 70-75mph driving on WV interstate highways I get about 20-21 MPG. If I just drop my speed to 65mph everywhere I go during a tank of gas I can reach 24 MPG. I've consistently seen those results out of at least the last 3 or 4 tanks of gas over the last couple months. If I take a US Route (speed limit 55) for 90 minutes to visit my parents my MPG goes up even more for that period of time because I'm going even slower than my usual 65-75 mph. I don't drive too much slower than the posted speed limit (5mph as I state above) because I don't want to feel like I'm crawling but just dropping 5 mph makes a noticeable difference in the range I can achieve with my tank (17.7 gallons). YMMV.

  • by Anonymous Coward on Sunday October 12, 2008 @07:14PM (#25348709)
    I have a realtime mileage display and variable cylinder technology in my car, and what I have noticed is that I can easily cruise at 75mph on 3 cylinders and get tremendous mileage in the process. However, when I hit an uphill grade, if I try to maintain 75 the other cylinders kick in and my mileage drops dramatically (to roughly 2/3). But, I have noticed that if I gradually back off on the accelerator while climbing the grade, bleeding down my speed to keep those other 3 cylinders from turning on, I can climb the hill while maintaining my high mileage. I've learned also to accelerate slowly on level and mild up grades (like near the top of the grade) without the other cylinders engaging. Obviously when going downhill I take full advantage and build my speed back up while still getting great mileage. Perhaps something like this is what you are observing? BTW, I don't play these games in heavy or rush hour traffic; I only use these techniques when traffic is light.
  • by Cordath ( 581672 ) on Sunday October 12, 2008 @07:14PM (#25348713)
    There are lots of little things you can do to save on gas. Many center around efficient stopping.

    For example, if I see a red light coming up, I'll often ease off the gas and coast in rather than maintaining speed and then braking near the light like most people do. In addition to saving gas on the way to the light, if the light turns green before you stop then you've also saved the gas it would have taken to accelerate back up to speed.

    This tactic can be quite entertaining if, for example, an impatient bozo in a SUV comes up behind you while you're coasting, honks, pulls around you and speeds ahead only to stop at the light, and then you smoke him as you coast through the light just as it turns green.
    • Mod parent up. (Score:4, Insightful)

      by seeker_1us ( 1203072 ) on Sunday October 12, 2008 @07:37PM (#25348891)
      While wind resistance scales with speed squared, the simple fact is that most of the energy wasted in a car is in stopping, not wind resistance. Normal driving around the city I can get 19-22 MPG, and I use smart braking like the parent discusses. Driving 65-75 MPH across states (where I am just GOING), I can get almost 35.
  • by 5pp000 ( 873881 ) on Sunday October 12, 2008 @07:15PM (#25348721)

    In a small, aerodynamic car, speed doesn't matter that much. (In a larger vehicle and especially trucks, with their poor aerodynamics, speeds above 60 do start to affect mileage more strongly.)

    But how vigorously you accelerate can make a big difference. In the worst of the gas price spike I made a point of accelerating gently and shifting much earlier than usual, and found my mileage improved by 15%.

  • by Ydna ( 32354 ) * <andrew@swe[ ].net ['ger' in gap]> on Sunday October 12, 2008 @07:33PM (#25348867) Homepage

    I'd say the way people blast off from the green light like their in a Formula 1 Grand Prix* is probably doing a bigger number on fuel economy in city driving more than anything else.

    * or not if you were Hamilton yesterday.

  • by hazem ( 472289 ) on Sunday October 12, 2008 @07:50PM (#25348967) Journal

    This is just another case where people don't realize (or care) that trying to maximize the performance of one part of the system (their commute) ends up diminishing the performance of the overall system.

    Only a few people doing this slow driving will result in large numbers of other driver stuck waiting at more lights. Even worse, this kind of slow driving will result in some other drivers driving recklessly trying to get around the slow drivers. It won't take many crashes, injuries, and deaths to completely wipe out any savings made to the economy by a few people driving slowly (if only from traffic backups due to crashes).

    Using these kinds of hypermiling techniques are just fine for an individual who doesn't have any regard for how their behavior impacts others.

    • by mcrbids ( 148650 ) on Sunday October 12, 2008 @08:08PM (#25349101) Journal

      Your comments implying the driving slower may be more dangerous is laughable - like the tales told of people who got into accidents while trying to buckle their seatbelt.

      As the average speed of the US driver has climbed, the death toll has risen as well - both in absolute numbers and in average deaths per mile travelled. There is no evidence that driving slower is more dangerous, notwithstanding your own personal feelings in the matter. And if somebody driving slow in front of you is enough to make you drive in a risky manner, you really shouldn't be driving, should you?

      • You sure about that? (Score:5, Informative)

        by rantingkitten ( 938138 ) <(kitten) (at) (> on Monday October 13, 2008 @12:11AM (#25351017) Homepage
        This study [] commissioned by the US DOT says otherwise, as do thousands of engineers across the country. I personally find this an interesting if dry read, because it's pretty damning evidence that speed limits are set artificially low for revenue generation purposes, since it can be demonstrated that posted limits have a negligable effect on how fast people actually go. Anyway, some things of note:
        • Accidents at the 58 experimental sites where speed limits were lowered increased by 5.4 percent.
        • Accidents at the 41 experimental sites where speed limits were raised decreased by 6.7 percent.

        The logic is that the majority of people are going to drive at a certain speed on any given road regardless (the "85th percentile" rule) and the one doofus going significantly slower than this becomes a very unexpected, slow-moving obstacle which requires people to either hit the anchors suddenly, or attempt to swerve around, both of which are clearly unsafe behaviors.

        While most cops won't care about this excuse because they want to maintain a ticket quota, many judges will, assuming no other violation and a good attitude, accept the "I was just keeping up with traffic" line as grounds for dismissal or reduction of a citation. There's a reason for this.

        I grant you that this study, and some others like it, mention only accidents and do not discuss or even mention fatalities, but the reduction of total accidents when everyone drives at the 85th percentile is a pretty clear fact. If everyone drove slower this probably wouldn't be the case, but since we aren't going to change the rset of humanity's driving patterns, telling people to drive slower than they should is dubious advice.

    • Mod parent wrong (Score:4, Insightful)

      by tpz ( 1137081 ) on Sunday October 12, 2008 @08:28PM (#25349247)

      Hypermiling isn't even remotely about slow driving. It is about accelerating at an optimal rate, cruising at an optimal rate, and carrying no more speed than necessary to get to the next known stop.

      Pay special attention to that last one. Carrying no more speed than necessary to get to the next known stop. A hypermiler's behaviour isn't going to affect anyone. If they were all going to be stuck at the next red light, they were all going to be stuck at the next red light. If they were going to make the light, everyone can cruise at their optimal rate.

      A hypermiler's behaviour only impacts how other drivers _think_ they are doing in terms of making good time to their destination. Such other drivers love to do things like see that a light is turning red and then _accelerate_ towards it because they want to be first in line. Or because it just feels good. Or whatever. But they'll be waiting at that exact same red light as everyone else, including the hypermilers.

      Posts like yours place the blame here on the hypermilers, but the blame should reside elsewhere.

      • Re:Mod parent wrong (Score:4, Interesting)

        by hazem ( 472289 ) on Sunday October 12, 2008 @08:56PM (#25349495) Journal

        Posts like yours place the blame here on the hypermilers, but the blame should reside elsewhere.

        Traffic is a system. I'm not blaming just the hypermilers. I'm blaming anyone who intentionally strives to optimize their own trip while not giving a damn for how that impacts the system as a whole.

        I've written and used simulations on traffic and it's pretty easy to demonstrate that one slow driver (one who's slow to accelerate, drives below the speed limit, and/or decelerates slowly) at the head of a pack of traffic will impede the flow of traffic for the entire pack causing the cars behind be stopped behind more lights and spend more time waiting at lights. That one driver may experience better gas mileage but it's at the cost of all the other drivers.

        That doesn't even take into account the psychological aspects where the behavior of the slow lead car can result in greater irritation of the drivers behind him and probably erratic driving on the part of one or more of them. If you're going to say that slow driver bears no responsibility in this, then you must also accept that the guy who races up the right hand side and merges late, causing a pile-up behind him also bears no responsibility for the crashes and carnage behind him - for clearly it was the other drivers who didn't respond properly. /sarcasm

        A hypermiler's behaviour only impacts how other drivers _think_ they are doing in terms of making good time to their destination.

        This is not correct. Let's assume in a case it takes the slow guy 20 seconds to get "up to speed" once a light turns green and the average driver 10 seconds to get up to speed. That slow driver has "eaten up" 10 seconds of the next green light. Had he not been in the way, 10 more seconds of cars could have made it through the next light before it turned red. That means 10 seconds of cars now idling at one more additional light.

  • Don't be aggressive (Score:3, Interesting)

    by hansamurai ( 907719 ) <> on Sunday October 12, 2008 @08:00PM (#25349045) Homepage Journal

    The best hypermiling technique I've found that anyone can do is don't be aggressive on the road. This is pretty obvious but I used to drive like a jerk and weave in and out of cars, constant slamming on breaks and jamming the accelerator. Then gas hit $2.50 and I had a baby on the way so I dramatically changed my driving habits. I coast a ton and never tailgate (well, I do draft behind semis sometimes on the highway). My MPG has gone up a ton and I was basically paying the same at $2.00 and $3.00/gallon for a tank of gas. I do mostly city driving so it's tougher to keep a constant 55 MPH (seems to be my optimum speed), but I just don't drag race from light to light anymore.

  • by i ( 8254 ) on Sunday October 12, 2008 @08:20PM (#25349193) accelerating relatively fast to something like 70mph,
    then pull of the engine and roll with no gear until You reach
    something like 10mph when You start the engine and repeat.

    This is the empirically show best method.

    But it will probably irritate other drivers...

  • by fermion ( 181285 ) on Sunday October 12, 2008 @08:37PM (#25349311) Homepage Journal
    There are two factors, I believe. One is highway driving where the car quickly reaches cruising velocity and the dominant power consumption, average over time, involves the energy necessary to keep the car at the constant velocity, i.e. overcoming friction. Such driving usually involve reletively constant velocity over a several or even tens of miles Under these highway conditions, there is generally a vertex in which fuel consumption is maximized. In the graph provided by the poster, this speed is between 50 and 60 miles per hour. If one just wants to go fast, and the argument is not about maximum fuel economy, then one can go 70 and the difference is not significant.

    But 45 miles per hour does not imply highway driving. It implies driving where the car must stop every mile or so. In this case the energy distribution is different, the dominant term probably being the energy needed to accelerate the car to cruising velocity, which, at 40 miles per hour, with 1 mile stops, occurs perhaps every two minutes. The energy of a car moving with a mass of 'm' moving at 'v' miles per hour is on the order of mv^2. This means that accelerating a car to 45 miles per hour will require twice as much energy as a car that is kept under 35 miles per hour. Now if one is talking about a small car traveling less than 25, and big hemi traveling at 45, then we are talking 4 times as much energy to accelerate the car every few minutes. Of course with a hybrid car some this energy is recovered, but then the rate of acceleration is factor. The faster one accelerates, the less adiabatic the operation, and the less energy is recovered.

    So to summerize. In the city, a hemi truck accelerating to 45 miles per hour requires maybe four times as much energy as compact traveling at 30 miles per hour. This energy directly relates to fuel consumption. On the highway where velocity is constant, the domanant factor is merely the energy to overcome friction, which primarily depends on how the engine is constructed and how the shape of the car interacts with the environment. This will probably be slightly different for every car, and every driving style. Thirty years ago it appeared that cars were built to go 80 miles per hour for maximum efficiency. I think it is criminal to drive a Porche slower than that. At the end of the day, for highway driving, it would probably be best to monitor the tachometer for optimal fuel consumption rather the speed. For city driving, slow accelerations with higher speeds only on longer stretches or road.

  • Best practices (Score:3, Interesting)

    by Todd Knarr ( 15451 ) on Sunday October 12, 2008 @10:00PM (#25350045) Homepage

    What I've found is there's two sets of best practices, depending on the type of driving.

    1. Highway driving, dominated by long periods of cruising. With modern aerodynamics, air resistance isn't usually a problem for passenger cars at posted speed limits (up to 75mph). SUVs and trucks have issues, but if you're interested in fuel economy changing to something else is the single biggest fuel savings you can get. Fuel consumption then's determined by two things: how efficiently your engine's turning fuel into power, and how many RPMs it's making. The first you can find by looking at a graph of your engine's power band (power produced vs. RPM). It's a plateau with a drop-off at either end. You want to stay in the plateau region, if you let the RPMs drop too far or climb too high your engine's burning more fuel than it needs to to generate power to keep you moving. The second's mostly determined by what gear you're in. So you want to maintain the speed that keeps you at the low end of the power band in the highest gear you have available. Any slower than that and you need more throttle (and more fuel burned) to maintain speed, or you have to drop into a lower gear and increase your RPMs (which means burning more fuel).

    2. City driving, dominated by acceleration from stops. Speed has a small effect, but the biggest fuel burn you have is accelerating away from a stop light. So adjust your speed to match the interval between lights as closely as possible. If you find lights going green just after you've stopped, slow down a bit. And if you find them going red before you get there, speed up. Going faster may burn more fuel, but starting from a dead stop burns much more so you save by avoiding the stop. And don't lolly-gag on the acceleration. You don't want to peel out, but you want to get up to speed fairly quickly so you spend the least time in lower gears. Remember, the lower the gear the higher the RPMs at a given speed and the more fuel you're burning. Plus, getting up to speed smartly makes it easier to judge the speed you need to maintain to hit the next light while it's green. Spend too much time accelerating and you'll either have to hit a much higher speed or you'll miss the next green, have to stop, and burn all that fuel accelerating again.

  • by iwein ( 561027 ) on Monday October 13, 2008 @06:11AM (#25352939)
    I tried to get optimal efficiency in a Fiat punto (turbo diesel).

    I noticed that at sane angular velocity there is a peek in efficiency when the turbo kicks in. However, if you go all the way down and let the engine run stationary in fifth gear you can get to a much higher efficiency. I managed to get twice the specified efficiency. The car will be running around 12.5 m/s then (which is about 25 knots)

    What happens is that because of the low drag at that speed, the momentum of the car is enough to keep the engine turning above the fuel injection threshold without help. Then the computer decides to stop fuel injection. The result is that the cylinders fire only once in four roughly. Almost any diesel car should be able to do this, as they put way to heavy engines in them.

    It won't surprise you to read, that you shouldn't tell your dealer, nor try this on the highway (they have a lower speed limit too).

    Please don't ask for help converting this to nautical miles per pint.

  • by MickLinux ( 579158 ) on Monday October 13, 2008 @07:32AM (#25353355) Journal [] There are a lot of tips on how to save mileage there... some of which are discussed in the comments of this slashdot story.

"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." -- Bertrand Russell