Well, with gas prices approaching $4 all over the USA, this may be an interesting topic to talk about. I tried to put together all my experience and knowledge, and came up with the list of tips with explanations. They do tell you why every particular advice helps or does not help you to achieve best gas mileage.
Give it a try, I am pretty much sure you'll see a noticeably better gas mileage if you do - and no gas savers needed
Last edited by Misha on Mon Feb 02, 2015 3:14 am; edited 3 times in total
Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA
Posted:
Sun May 11, 2008 11:50 am
Rodney submitted a feedback:
Quote:
I have to comment on your idea of accelerating down a hill and not accelerating up a hill to save some gas is just outright a wrong thing to do. Not only does it not save you any gas it is dangerous and your going to make people who are following mad and have road rage.
Well, I do agree absolutely that you need to think of other drivers around and consider how they react to what you do on the road. This is a golden rule of road safety, and I never meant to not follow it. If you read the original advice carefully, it specifically says:
Quote:
Of course you have to coordinate this with the traffic flow.
Yeah, that pretty much means - if you are in traffic, don't do this. No gas saving benefit justifies increasing the risk of accident.
Now, about the benefits part. I still think I am right. Again, I might not have been very clear. I am not talking about stomping on gas downhill. I am talking about not applying brakes (including engine braking) when you go downhill, so inertia does accelerate your car and then help it to climb uphill. You want to either switch to neutral, or maintain the same position of the gas pedal. Yes, you will most probably go over the speed limit - but I am not a big fan of speed limits anyway. However, I do not mean to drive faster than you deem safe. Again, no gas saving benefit justifies increasing the risk of accident.
Rodney, I hope this clarifies the issue. If not, I'm open for further questions.
Joined: Oct 05, 2008
Posts: 79
Location: Albuquerque NM
Posted:
Sun Oct 05, 2008 5:20 pm
Several of the tips for economical driving are just plain wrong. At one time, I worked for a manufacturer of industrial engines and generators. I also took a course on internal combustion engine design, and I know how to maximize economy.
Tip 6 "Do not fill your tank up completely" is wrong if you have to drive out of your way to fill the tank, in which case the extra driving to get gasoline more frequently will use more fuel than the reduced weight. In any case, the amount of fuel saved would be very little.
Tip 8 "Try not to stomp on the gas anymore than you need to" requires more explanation. If you accelerate too gradually, you will spend too much time in the lower gears. Maximum fuel efficiency is not obtained until you are in the highest gear.
Tip 21 "Should you avoid excessive idling?" The fact is that starting the engine shortly after stopping it uses very little fuel. If you will be stopped for more than 15 seconds, you can save fuel by stopping the engine. However, there are obviously other considerations and frequently stopping and starting the engine will reduce the life of the starter.
Tip 26 "Should I drive in a higher gear?" The statement that the engine is most efficient around the middle of its rpm range is misleading. That is true ONLY when the engine is running at full load or nearly so, which is rarely the case in a car. Many years ago, BMW tested their cars and found that the most economical way to drive, with a manual transmission, was to start out with a heavy foot, but upshift at only 2000 rpm until getting into top gear, then accelerating more gradually. The optimal speed at which to upshift would depend on the car. With an automatic transmission, the driver has less control, so it's best to accelerate at a moderate rate until reaching the speed at which the transmission can shift into top gear with the torque converter locked, then accelerate more gradually. In any case, accelerating too gradually is not fuel efficient.
A serious loss of efficiency on an Otto-cycle engine (4-stroke spark ignition engine) is the throttle valve. Any time the throttle valve is restricting power, there is a partial vacuum in the intake manifold. It takes power to suck the air in past the partially closed throttle valve, and that reduces efficiency by increasing pumping losses. That's why an engine is generally more efficient when the throttle is wide open (there may be a few exceptions, but in any case it is not efficient when running at light loads). About the worst thing you can do for fuel efficiency is to drive in a gear lower than necessary for the power you are using. Of course, you should not run the engine so slowly that it knocks or doesn't run smoothly, and the minimum acceptable engine speed depends on the design of the particular engine.
The main reason, but not the only reason, that a Diesel engine is more efficient than an Otto-cycle engine is that the Diesel engine does not have a throttle valve to cause pumping losses.
A car with a manual transmission will deliver better fuel mileage than a car with an automatic transmission, PROVIDED THAT THE DRIVER USES THE MANUAL TRANSMISSION AS ECONOMICALLY AS POSSIBLE!! Otherwise, the automatic transmission could deliver better fuel mileage than the manual transmission.
Tip 27 "Do you need to slow down to 55 mph to save fuel?"
The fact is that, in general, a car is most fuel efficient at the lowest speed at which it will run properly in top gear (with a manual transmission). With an automatic transmission, it is most efficient at the lowest speed at which the transmission will shift into the highest gear with the torque converter locked. For most cars, that speed would be less than 55 mph and for some cars, it could even be as low as 30 mph. The reason is that the power required to overcome air resistance increases with the cube of the speed, i.e., increasing the speed from 30 mph to 60 mph increases the power required to overcome air resistance by 8 times!! So, although the engine becomes more efficient when you drive faster, the increased power required to drive faster is enough to overshadow the increased engine efficiency.
Tip 7 "Fill you tank at the coolest time of day." In fact, the service station's tanks are underground, so the temperature of the fuel remains constant. Thus, it really doesn't matter whether you buy fuel at the coolest time of day.
The above said, many of the tips are correct, although some are of little consequence. For example, carrying heavy things in the trunk would reduce fuel efficiency, but it is impossible to carry enough in the glove compartment to make a measurable difference, unless perhaps you complete fill the glove compartment with lead or uranium.
A tip that should be added is, when possible, to take advantage of timed traffic lights to make it possible to drive at a constant speed. I see drivers constantly stopping and starting, or constantly slowing down and speeding up, when if they drove at the back of the pack, they could drive at a perfectly steady speed and save considerable fuel.
"Pumping losses are caused by the way power output from a petrol "Otto" engine is regulated. It is regulated by controlling, or rather constricting airflow to the engine. This constriction of airflow creates partial vacuum (low pressure) in the inlet manifold. Maintaining this "low pressure" in the inlet manifold wastes energy.
"One reason for diesel "Otto"-engines being more effective than the corresponding petrol engines is because there is no pumping loss in a diesel as power is regulated by injecting less fuel into the cylinders and not by choking the airflow to the cylinders."
Considerable work has been done on finding ways to eliminate the throttle valve. That can be done by having total control of the timing of the intake and exhaust valves and not using a camshaft, but rather, by controlling the valves by computer. Both Diesel and Otto cycle engines can be made more efficient, more flexible, and more powerful that way. However, it is not an easy thing to do, which is why camless engines are not yet in production.
Some years ago, International Harvester successfully tested a camless Diesel engine in a truck. Perhaps eventually such a system will be in mass production.
In any case, you can see from the article why an engine is more efficient at wide open throttle and why it is more efficient to accelerate with a heavy foot and upshift at low speeds. Actually, the penalty for upshifting at somewhat higher speeds is very small, provided that one uses a heavy foot. I fully realize that that is contrary to the watered down and over-simplified advice that one generally reads.
An exception may be that some older pre-fuel innection engines run with a richer mixture at wide open throttle, in which case 1/2 to 3/4 throttle may be more economical than full throttle before getting into top gear.
It is impossible to make acceptable stepped-ratio automatic transmissions that are as economical as manual transmissions because drivers would be very annoyed at having a sudden and obvious loss of power occur with each upshift. In fact, shifting a manual transmission as efficiently as possible is annoying to passengers which is one reason few drivers would do it while carrying passengers.
Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA
Posted:
Mon Oct 06, 2008 10:39 pm
OK, thanks for giving me some time and here we go
If you did not do so yet, I think you might find it interesting to read my short bio, it will qualify my credentials
Now, let's go one by one.
Tip #6. No question. Yes, I did assume that you normally fill up on your way. Thanks for pointing this out, I'll think how to rephrase the tip.
Tip #8. Well, this is definitely a big topic, and one has to stop somewhere in clarifying every tiny bit of it. I think the level of detail is right for the list of tips. And I don't agree with you last phrase. More precise, just partially agree. I think it is correct when you are talking about highway driving, but in city driving you never get to the highest gear usually.
Tip #21. Well, we could argue about exact time, I don't think it makes a material difference. And as you rightly noticed there are other considerations here, too.
Tip #26. You didn't convince me. Please link me to the BMW tests you are talking about. Yes, the Otto-cycle engine is more efficient on a full throttle, but this does not mean using full throttle in a complex cycle of city driving gives you any fuel saving benefit. And also, auto transmission has a built-in disadvantage of slightly lower efficiency.
Tip #27. Well, if you state this is the fact, please prove it. I don't see it as a fact, rather a speculation. And if I remember correctly, it is a square, not cube.
Tip #7. Yep, this one probably is the weakest, cause temperature underground does not change much. However it still does change, and all the piping and hoses are changing there temp with the environment. So, I would insist there is still some marginal effect, but this is for those who are really anal about saving gas
And thank you for the last tip - yep, this is definitely one that I missed
Now, to your second post - it does not change my stance on tip #27. This is the topic that no amount of speculation can prove or disprove. Only test data from a trusted source will do
Joined: Oct 05, 2008
Posts: 79
Location: Albuquerque NM
Posted:
Tue Oct 07, 2008 1:31 am
#8. Whether you get to use top gear in city driving depends on both the car and the speed of the traffic. I find that I can often use 5th gear, but doing so requires frequent downshifting as the traffic slows, and many drivers would not be willing to do the necessary extra shifting. The minimum reasonable speed for 5th gear on my Mazda 3 is about 30 mph / 1200 rpm. Obviously that would be too slow for many cars.
#26. The BMW tests were done before 1970, so it is unlikely that they can be found in the Internet. However, the principals of physics have not changed. Also, the tests were done with carbureted engines and carburetors often deliver a richer mixture when a heavy foot is used, so maximizing efficiency by accelerating with a heavy foot and upshifting at low speeds would be expected to be more effective with fuel injection. Another advantage that fuel injection has over a carburetor is that fuel injection has no accelerator pump. With a carburetor, an accelerator pump is necessary to coat the inside of the intake manifold quickly with fuel when the accelerator is depressed quickly, else the engine would stumble. With fuel injection, the injection occurs directly in the port so the need to make the mixture richer briefly is greatly reduced.
I did not see any figures to quantify the fuel saving resulting from using BMW’s method, so I can’t prove that the savings would be sufficient to justify using the method. However, it does make sense that it would work.
#27. The FORCE required to overcome air resistance increases with the square of the speed. Thus, if the speed is doubled, the FORCE required to overcome air resistance is increased by 4 times. However, if the force did not increase, the power required would be doubled simply because power = force times speed. Thus, the overall effect is that when the speed is doubled, the power required to overcome air resistance is multiplied by 8, i.e., it is cubed.
To see how FORCE (or drag) increases with speed, see the following:
The following web site shows that the power required does in fact increase with the cube of the velocity (a quotation from the web site follows the link):
“For high velocities — or more precisely, at high Reynolds numbers — the overall drag of an object is characterized by a dimensionless number called the drag coefficient, and is calculated using the drag equation. Assuming a more-or-less constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity. The standard equation for drag is one half the coefficient of drag multiplied by the fluid mass density, the cross sectional area of the specified item, and the square of the velocity.”
The following is a link to a graph which shows mpg vs speed for several cars:
You will notice that several cars get their maximum mph at around 30 mph. However, that is not true for all cars. For one thing, some cars with automatic transmissions will not lock the torque converter until the speed is greater than 40 mph. The strange curves may be at least partly the result of the shifting and locking of the torque converters of automatic transmissions.
I think that the above sources for #27 can be trusted.
It may be that the most effective way to improve fuel efficiency would be to increase the gasoline tax greatly and reduce the income tax to compensate. That would provide a greater incentive to drive more efficiently and to buy more efficient vehicles.
The most effective way I minimize gasoline use is to use my car as little as possible and use one of my motorcycles instead. My 2007 Suzuki SV 650 averages almost 60 mph and my 2006 Honda VFR 800 / Interceptor averages about 44 mpg. Of course, when riding a motorcycle, it is important to ride as defensively as possible and wear very conspicuous protective gear.
Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA
Posted:
Tue Oct 07, 2008 4:06 pm
I'll get back to this later today or tomorrow. For now I do agree on the cube, my mistake But the rest is more complicated, and I need time to put a reply together
Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA
Posted:
Wed Oct 08, 2008 1:49 pm
OK, I'm back to this
Let's first get clear on an issue of trusted sources, specifically using wiki as a one. I personally don't trust wiki on anything that can have more than a single answer. Just go look at their "driving" article, and you'll see how much unrelated and plain wrong information is there, and how much of needed info is missing. Talking of your uses of wiki - the first one is perfectly OK, you just used it to clarify a well known scientifically proven fact. The second use however is highly questionable and does not convince me a tiny bit. I can use excel and/or photoshop to draw even more exciting graphs that "prove" any point I want to and post them there - as long as I am friends with wiki crowd.
Our disagreement on items 8 and 26 is about driving style basically. As far as I understand, you advocate using a full throttle whenever possible and getting to the highest gear as soon as you can, regardless of your driving environment. You claim this style is the most fuel efficient. I beg to differ. This style definitely work on highway driving, here I am with you on that. It's pretty much like planes getting to the set height and speed as fast as they can, the same idea, and it seems to work for planes, and I tend to agree it should work for cars, too - but I never saw any serious research on this.
However, city driving is altogether different animal. I am not talking about driving on highways within city borders - it is still highway driving. I am talking about real city driving, which is neverending speed-brake cycle. And all my experience tells me I am going to use much more fuel if I have a lead foot in the city. I easily can change my stance on it any time if I see a result of serious research, cause subjective experience is just that - subjective experience, but so far I did not see any such research myself, and you did not show me one either.
Now, coming to the most economical speed to drive - #27. The pic in Wiki is really laughable. I bet it was manufactured to oppose the abolishment of country-wide 55 mph speed limit. It clearly attempts to persuade us that for most cars 55mph is the most economical speed. In reality it is much more complex issue, and the only real way to have a definite answer is to make a series of properly thought-through and prepared tests. No speculation can help here IMO, cause it involves too many partially interdependent variables.
Just for starters, if i remember correctly, the drag is absolutely insignificant on speeds like below 50 mph, so your statement that 60 mph will require 8 times more power than 30 is true only in the part of air resistance, which is only a part of a total resistance to overcome, absolutely insignificant at speeds below 50 or so. Another point, when you are talking about power, it does not translate into mileage directly. Remember, that power is per unit of time, and mileage is per unit of length (way?path?space?distance? - whateva ). Higher speed works to increase mileage to the extent that you cover more distance per the same unit of time, thus spreading the higher amount of fuel consumed over more miles. Which tendency overcomes - not clear until you test.
Anyway, I have to admit that I don't know the magic number. 60-65 is just my wild guess based on my experience and level of understanding I have now. Again, I am happy to change my stance - if you provide me with conclusive evidence. And no, speculations and wiki images do not count as such. Neither do youtube videos
Joined: Oct 05, 2008
Posts: 79
Location: Albuquerque NM
Posted:
Wed Oct 08, 2008 3:24 pm
Whether one can actually use a heavy accelerator foot and upshift at low speeds obviously depends on traffic conditions. When traffic is really heavy, it would obviously be impossible. However, it doesn’t result in the fierce acceleration that one might expect since using that technique, one quickly gets into 3rd gear and at speeds not exceeding 2000 rpm in 3rd, the acceleration isn’t very strong; the power drops off considerably with each upshift when shifting at such low speeds. Fortunately, I’m in a situation where I seldom drive in bumper to bumper traffic. That technique is most effective in light traffic in the city. On the highway, one usually isn’t stopping and starting anyway, so the shifting technique would be of little importance for fuel efficiency.
Regarding the most economical driving speed, when I was younger I often read automotive magazines, such as Road and Track, Car Life, etc. In their road test articles, they often printed the fuel mileage for several speeds. They tested that by connecting a glass container to the engine; the container was graduated so they could tell exactly how much fuel was used. The Consumer Reports used to do the same thing. In most cases, the fuel mileage decreased as the speed was increased above 30 mph. However, because cars are aerodynamically more efficient now and because they usually have automatic transmissions which will not get into top gear with the torque converter locked until the speed exceeds 30 mph, the most efficient speed would now usually be greater than 30 mph. However, it would still be unusual for the most efficient speed to be greater than 50 mph. One of the reasons given for not scrapping the 55 mph speed limit was that fuel efficiency drops off as the speed exceeds 55 mph. By doing a google search, one can find many different fuel efficiency curves; Wikipedia is not the only source of information.
As you correctly point out, at lower speeds, aerodynamic drag is not the principal source of rolling resistance. Other sources of rolling resistance are tires (although radial tires have significantly reduced rolling resistance), and friction in the wheel bearings and other moving parts. I actually stated that the power required to overcome AIR resistance is 8 times greater at 60 mph than it is at 30 mph. That does not mean that the TOTAL rolling resistance is 8 times greater; it isn’t, the reason being that the other sources of rolling resistance are more nearly linear than the air resistance. In any case, above a certain speed, the total rolling resistance increases faster than the engine efficiency increases, which is why fuel efficiency drops as the speed increases above a speed which depends on the design of the car.
The technique to get the best possible fuel efficiency is one that cannot be recommended except in case of emergency; it works only with a manual transmission. It was well known in earlier times and has been demonstrated to be highly effective. The resulting fuel mileage can be spectacular. The technique is to get into top gear as soon as possible then, with the accelerator down to the floor or nearly so, accelerate to about 50 mph. Then, throw out the clutch and shut off the engine. When the car has coasted down to the lowest speed at which the top gear can be used, turn on the ignition, engage the clutch, and again accelerate to 50 or so. During this repetitive procedure, the accelerator position should not be changed, at least with a carbureted engine. Again, this procedure cannot be recommended except under highly unusual circumstances, but it does work.
Joined: Oct 05, 2008
Posts: 79
Location: Albuquerque NM
Posted:
Wed Oct 08, 2008 3:27 pm
P.S. - My sister and her husband, to save fuel, often limit their speed to 50 mph when they can do so without disturbing traffic. They tell me that they actually do get better fuel mileage by doing that, and that the mileage sometimes even exceeds 40 mph with their Saturn. They are both technically competent, so I assume that they know how to measure the fuel mileage accurately. Of course one example is not proof, but even so it is a good indication.
the most common mistake by new and experienced drivers alike is the acceleration procedure.
When the light turns green nailing the accelerator on your car or truck, and getting that adrenaline rush as you're pushed back into the driver's seat is a lot of fun, but it's also an expensive kick.
When you look at what it does to your average gas mileage. Jackrabbit starts cause the engine to literally suck the petroleum in large amounts, Rather than in taking fuel in a smooth, civilized manner.
While we're all guilty of engaging in the stop light drags, next time you feel the temptation, ask yourself if it's really that important to show the guy in the Mustang, who the real king of the road is. Remember, it's your wallet, so why is it so important to impress a total stranger.
Keeping your highway speed and 55 mph can increase fuel economy as much as 25%, compared to 75 mph. Keep in mind that over 50% of the energy required to move a vehicle down the road is spent overcoming aerodynamic drag.
The faster you drive, the more air, you will have to push out of the way. And this dramatically increases rolling resistance. Consequently, the fuel economy decreases rapidly at speeds above 60 mph.
Joined: Oct 05, 2008
Posts: 79
Location: Albuquerque NM
Posted:
Fri Dec 25, 2009 2:38 pm
Although it's a minor point, I prefer to refer to driving technique rather than driving habits. That's because it's assumed that habits are something well ingrained and very difficult to change, whereas a technique is easier to change.
The most economical amount of acceleration is not the same for both manual and automatic transmissions. With a manual transmission, if you use a heavy foot when the light turns green but use as little clutch slippage as possible and upshift at the lowest reasonable speeds, you will not get neck-snapping acceleration since the engine will never reach a speed at which it provides high power. And, that is the most economical way to use a manual transmission. If you accelerate too gradually, you will spend to much time in the lower gears and you don't achieve maximum fuel efficiency until you get into the highest gear.
With an automatic transmission, there will be more slippage in the torque converter if you accelerate with a heavy foot, and that wastes fuel. Also, the transmission will upshift at higher speeds. With an automatic transmission, you have less control. So, with an automatic transmission, it is probably best to accelerate at a moderate rate, until you reach the minimum speed at which the torque converter will lock, then back off on the accelerator to cause the torque converter to lock. But even with an automatic transmission, accelerating too gradually is wasteful because the transmission will spend more time in the lower gears.
If you have a car that can easily win drag races, you have already made a mistake if good fuel mileage is important to you. The first step in getting good fuel mileage is to get an economical car. Aside from that, the two worst things you can do for fuel mileage are to drive at high speeds and to drive in a lower gear than necessary for the power you are using.
It's OK to drive 55 when the speed limit is 75, but only when traffic is light and drivers who want to go faster can easily pass. But it's NOT OK to do that when it disrupts traffic. The most economical speed depends on the car, and for some cars, that could be as low as 40 mph. However, I doubt that the most economical speed would ever be greater than 55 mph. But if you are driving on a long trip, the fuel you save by driving only 55 could be offset by having to stay at a motel an extra night and spending more for meals on the road.
It greatly helps to hit lights on green whenever possible. I often see people driving at a higher speed that what the lights are timed for, so they have to stop at every light. I just float along near the end of the pack, keep a steady speed, and hit the lights on green. Choosing the route can make a difference too.
The foremost thing in saving gas is to maintain speed level while shifting gear. During a long drive maintaining speed level between 55-65 mph is the more efficient way in saving gas.
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