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Fuel Efficiency and Slow Driving?
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 truckandbarter.com 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?
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.
Serious? Seriousness is well above my pay grade.
But I've noticed I get less tired if I walk rather than run :-)
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.
No sig today...
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.
Using the holy grail of OSes...
Do like everyone else does, drive about 6 inches behind me at 65 mph.
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.
this nation, under God, shall have a new birth of freedom. -- Lincoln, Gettysburg Address
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.
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.
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%.
Your god may be dead, but mine aren't!
Slow acceleration is worse than (reasonably) rapid acceleration. The rest of your tips I can't argue against, but I do know that decent acceleration outside of the red zone is better.
Yeah. There was an article in Readers Digest a few months ago about "hypermiling" or whatever, and it was a case study in "I'm the only person on the road". Incredibly arrogant and self-centered.
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
Oh, for mod points. Most people (well, most men anyway) are competitive, and we like to beat our "high scores". Tachometers show us speed, clocks show us time, but neither of those contributes to efficiency. Adding a fuel economy display gives a better goal to beat.
If you were blocking sigs, you wouldn't have to read this.
Buy a Scangauge 2. Google it. It costs around $150. Hook it up to your car, and it will show you shitloads of fuel economy information and more.
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?
Drive downhill.
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.
"The great thing about multitasking is that several things can go wrong at once." -me
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.
If someone has already said cruise control, I apologize for restating it, but for me it works. The computer in most modern cars can control the speed of the car within a 20th of a mile. (1/20) That is a lot closer than any human can handle. This prevents you from over/under speeding (I.E. You wanting to go 70 MPH but you waiver between 68 and 72). There are other obvious things you can do though: regular car maintenance being the biggest of them all
I always thought those people were assholes, and I'd fly into a rant about how dangerous and reckless that behavior was. But they're just trying to save money. People really are basically good after all!
Oh, yeah, it's not easy to pad these out to 120 characters.
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.
Postet this before and got heat for the turning off down-hill. I am doing it where possible and I feel like - you can do what you want, not my business.
There is no question that coasting with engine off uses less fuel (zero) but there are other legitimate questions:
Is it necessary? - in many modern cars the fuel is cut off while engine breaking, so in that case you are using zero or thereabouts fuel anyway. On the other hand any savings on fuel (if any) can be easily offset by the extra wear on the brakes. You could also cause major damage to your transmission if you shift into a low gear or even into reverse by accident while moving at high speed.
Is it safe? - No. It's a bad idea to drive in neutral as you cannot quickly accelerate should the need arise, and also increased strain on the brakes makes it more likely that they will fail at the wrong moment.
Is it legal? - Depends where you live, in California and probably some other states it is actually illegal - look up California Vehicle Code 21710
Negative moral value of force outweighs the positive value of good intentions.
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.
Reviewing just the first hour of video games.
..is 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...
Mundus Vult Decipi
Air resistance on reasonably aerodynamic objects is actually more like |v|^1.4, not v^2. Air resistance for objects like bricks (or trucks) is roughly v^2.
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.
Oh yes. My car's got one of those displays and my fuel efficiency is about 10 MPG higher than it was when I first got the car, because I pay attention to how my driving affects efficiency.
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.
"She's a scientist and a lesbian. She's not going to let it slide." Orphan Black
I second your remarks. If prices were allowed to rise according to supply and demand, then companies would make sure gas got to NC, thus lowering prices.
Tachometers show us speed
Does your car only have one gear?
That's not an acceptable answer for most. Plus, you can get the same fuel efficiency and still drive. You just have to make one or two small compromises.
Essentially, the idea is to only drive downhill by letting go of the brakes and coast the rest of the way. If you run out of sufficient momentum to carry you along, or want to go uphill, you just have to push the car for a bit.
So, you just choose your workplace at a higher elevation than your home and push your car up in the morning after a hearty breakfast. Then, at the end of the day, you just coast down back home. Easy.
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.
I've got a Honda Civic Hybrid. And have I ever gotten to love the mileage!
Your mileage may be great, but your smug emissions are off the charts.
Bring back Sirius Punk!
Highway patrol in Seattle have started ticketing people for driving too slowly (hypermilers) and rightfully slow. I wish more cities/municipalities would start doing this, cause it's irritating as hell and completely useless. You get worse mileage the slower you drive. Please drive the posted limit. (It's safer for everyone)
Think binary! You need a 1 MPH brick, a 2 MPH brick, a 4 MPH brick, etc. Just build up a stack.
Stop! Dremel time!
The throwout bearing is also known as the Jesus bearing to those who wrench on cars. Usually after rebuilding the engine, installing it in the car, and topping up the fluids, you'll notice the Jesus bearing sitting on top of the toolbox.
Air resistance still goes like v^2, there's just a different coefficient of drag in front of it depending on the aerodynamics.
Stop! Dremel time!
I have an 2001 Sentra and just inst hooked up a gadget I got from Think Geek (ScanTool, I believe its called) that reads the engine computer through the OBDC2 connector. I can verify that taking my foot off the gas does shut off the injectors if the car is in gear and going fast enough. From the ScanTool manual I infer that this behavior is common, but not universal among cars.
The biggest problem I see on the roads are large cars being driven around with only 1 person in them! How "fuel efficient" is that?
About 9 months ago I got sick of paying high prices for petrol and wanted to do my bit for the environment, so I got my motorbike license and purchased a 125cc scooter. I haven't looked back. Now my wife and I only have two vehicles, a small 2.0 litre car and a 125cc scooter. These two vehicles are fine for everything we need them for.
While I admit scooters aren't right for everybody, they are definitely a possibility for the majority of people. When I try and encourage other people to do their bit and buy a scooter they often come back with the following responses...
"They don't work out that much cheaper once you buy everything." - Complete BS, sure there's a little bit involved in the outlay (Scooter AUD$3000, license/training AUD$500, gear AUD$500, rego AUD$350, comp. insurance AUD$200) but once you're up and running these things run on the whiff of an oily rag. Consider it an investment. Right now I'm doing about 150km per week to and from work (and a little bit of running around on the weekends), I'm averaging about 30km per litre! It costs me around AUD$9 per week to fill up, that's with 98RON premium mind you! So you do the sums and see how much it will save you.
"Scooters are dangerous, you could get killed." - Again, complete BS. I was sceptical about how everyone kept saying how dangerous riding a scooter would be. So I did my research and contacted my state's Transport Authority. The stats were even surprising to a sceptic. In the past four years in my state there's been one fatality on a scooter and only three other fatalities on motorcycles less than 250cc. Once you go over 250cc, people seem to become retards and there are a lot more deaths. Mind you, 2/3 of those were with stationary objects i.e. poles, parked cars etc. People forget too that on a scooter you're a much smaller target and much more agile. The main risk I've faced seems to be people coming into my lane without checking blind spots. In all cases I've been able to successfully swerve/break/accelerate out of the way.
"What about riding in the rain, you'll get wet." - No, you don't if you have the right gear. I bought a weatherproof clothing straight up. I wear this gear over my normal business attire with leather shoes and I'm yet to get wet. I've even ridden in hail and I didn't feel a thing!
"You can't carry much stuff on them." - This is the only place where scooters fall down slightly. But still, scooters offer a few storage options. Most come with storage under the seats - big enough for a bag or your helmet. There's usually a bag hook to hang some shopping bags, you can get a top box put on the back for more storage and I also wear a back pack.
"They're too slow and therefore dangerous." - This is true for the 50cc scooters, which I believe are only suitable for inner city riding. But my 125cc will top out at about 100km/h and will happily cruise at about 90km/h. They are also *very* quick off the line.
Did I also mention they're very FUN to ride?
So what's your excuse?
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.
mirrorshades radio -- darkwave, industrial, futurepop, ebm.
I actually took an upper division Physics course called "Physics of Energy Conversion and Usage". About half the class was on fuel economy in cars. Here's what it all comes down to:
City Mileage:
What matters most is how light the car is. You're stopping and starting all the time, so you're re-accelerating all that mass each time you start and then dissipating it as heat in the brakes when you stop (unless you have regenerative braking, which still isn't all the efficient). The second most important thing is how much energy you waste while idling. A big displacement engine needs more gas just to sit there at idle than a small one does (of course, this doesn't matter if the engine shuts off automatically at a stop like in a hybrid). Aerodynamics don't matter around town as wind drag is small compared to rolling resistance and overcoming the inertia each time you leave a light.
So, a light car with a small engine gets good mileage around town.
Highway Mileage:
Here what matters most is aerodynamics, wind drag goes as about the square of the speed and rolling resistance only scales up pretty much linearly. Once you're up to cruise speed, it doesn't matter if you weigh 1,000lbs or 10,000lbs -- you already have the car up to speed so weight no longer matters. The most efficient speed will depend on the aerodynamics of the car. A brick shaped car will have its aerodynamic drag dominate the rest of the equation at a much lower speed than a slick shaped one will.
The factors that go into wind resistance are:
1) cross sectional area, this scales linearly. Double the cross sectional are and you double the wind drag.
2) drag coefficient, this is basically how slick the car is (spoilers in the right places etc.) Note that you can tune this to work best at a certain speed, if you want. That is, you can make the car most "slick" at 55 or 65 or 75 by design.
3) speed -- for the speeds we're talking about, the drag goes pretty much as the square of the speed (it goes way up as you approach the speed of sound, for example), but basically as you double the speed you quadruple the wind drag.
So, a small car (cross sectional area) with good aerodynamics gets good mileage on the highway.
One more thing that matters to both city and highway mileage is what % of the time your car can run at wide open throttle (WOT). Engines are most efficient at converting fuel to energy at WOT, any throttle setting lower than wide open causes the engine to suffer a lot of inefficiencies, mostly in the intake manifold -- the car is sucking air/fuel in through a straw and putting a lot of energy into doing so. It's like a backwards turbo charger. So, what you want is a weak engine that can run at wide open throttle at your highway cruise speed and off the line around town (and then shut it off when you stop). This means, however, that your car is going to suck performance wise and why econo-boxes suck to drive -- the engine has to be *just* strong enough to get the car off the line w/o holding up traffic and able to get it to highway speed but no faster (no passing, unless you're going downhill).
So, what is the most efficient speed for highway mileage? IT DEPENDS ON THE CAR. Of all the variables above, the only ones that vary as speed does are:
1) the aerodynamics of the car: for what speed did the car designers optimize the aerodynamics?
2) the size of the engine: the more powerful the engine the more likely it is to have its efficiency peak at a higher speed because you're closer to running it at WOT (will still get worse mileage at any speed than a less powerful engine, mind you).
I've done enough physics homework to not give a crap about how light my car is. I want a heavy car so when the Ford F250 running late to a job site blows the light and comes through my passenger door I have something to contribute to my half of the momentum-transfer equation, and enough body rigidity and safety features to keep me intact (both of which add to the weight).
My favorite quote doesn't fit into 120 characters. Now no one will like me.
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 going downhill south from Harrisburg, PA to Charlotte, NC for over 400 miles without refilling -- cruise control and standing on the clutch to slow entering town areas with lower speed limits.
It is because our cars are cooler. The low drag is because we are not driving a block on wheels. The V8 engine and 6-speed transmission allow us to accelerate well and coast without going over 2000rpm.
Do you get better mpg because the West Coast is flatter? In the Philadelphia area, we rarely see a half-mile of road because a hill blocks the view.
In 2007, I did little highway driving and averaged 16mpg, never 280 miles between refills. In 2006, I almost reached 23mpg for a few tanks, but still only averaged 17.5mpg for the year. TransAms are not good if you care about fuel economy. [I don't. The '99 was traded after 3.5 years with 54K miles -- much driving for well-paid consulting work. The '02 is 6.5 years old and just passed 50K miles -- much working from home. I still enjoy all-day pleasure drives.]
I spend my life entertaining my brain.
Astonishing that I haven't seen anything about that in the comments (though I haven't read all of it yet): make sure to have appropriate tire pressure can make a huge difference as well.
Go with at least the suggested pressure, maybe even a little (up to 0.5 bar) above and you will notice a difference in fuel consumption too.
Don't get over 0.5 bar more than the suggested pressure because it may effect the tire grip in a bad way.
This will affect your fuel efficiency no matter if you drive fast or slow.
Having recently ditched our fuel eating Saab 95 estate (automatic) that averaged 22-25mpg, we've now got a Saab 93 Sportwagon diesel (dreadful name, but nice car)
We've been testing it's mpg a lot recently. Town driving it gets 35-40mpg depending on conditions and traffic. This at speed averaging around 30mph. On Motorways at 70mph, consumption is about 40-42mpg.. at 60mph we easily get 51-53mpg.
The economy savings between 70 and 60mph are quite considerable. At £1.20 a litre (for diesel) that makes quite a difference on a long journey.
I always believed that cars optimum fuel economy had gotten better at higher speeds, but that's clearly not the case in my Saab.
it might be a Vauxhall/Opel Vectra underneath, but it's still a nice car to drive!
A couple of years ago jeremy clarkson did just this test. He found the following... Low rpm http://www.autobloggreen.com/2007/12/26/video-london-to-edinburgh-and-back-on-one-tank-of-diesel-in-an/ enjoy!
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.
Show a man some news, distract him for an hour. Show a man some mod points, distract him for the rest of his life.
It's not in the speed you drive, it's in the way you drive. It's in little things like:
- Don't rev the engine above 2500 rpm, change gears faster.
- When a traffic light ahead is red, take your foot from the accelerator. Modern engines use no (that's right: NO) fuel on engine braking.
- Anticipate to the traffic ahead, e.g. give room for somebody so you can slide into his spot.
- Don't start the engine until you are ready to set off.
And this will absolutely NOT make you drive any slower, but I've seen tests where they showed you could save somewhere in the range of 6 to 15% fuel.
Here's a nice link to a Dutch programme called "the new drive":
http://www.hetnieuwerijden.nl/english.html
http://www.rd.com/your-america-inspiring-people-and-stories/ease-on-down-the-road/article55921.html There are a lot of tips on how to save mileage there... some of which are discussed in the comments of this slashdot story.
Correct Horse Battery Staple: 72 bits of entropy. Enter "Correct H" into google. When it generates the phrase, that's
All of your concerns are addressed in Improve MPG: The Factors Affecting Fuel Efficiency which was Slashdotted more than two years ago.
In a nutshell, yes, the horsepower requirements increase with added speed as your engine must fight with increased drag. So driving slower mitigates the energy required to overcome the additional force.
HOWEVER, going too slow and you don't cover enough distance for the amount of gas you use. In an extreme example, idling at 0mph is an asymptotic point on the mpg graph. All cars have different "sweet spots" where they are optimal. Those "sweet spots" are typically dictated by particular RPM ranges and the corresponding speeds by which a chosen gear will drive the car within those RPM ranges.
Some people will say that simply driving in the highest gear at the lowest speed will always produce the best mpg. Not necessarily so (though generally so). As an example, a 2006 Jeep Wrangler turning 35" tires gets exactly the same mpg in 6th gear as it does in 5th gear at 75mph. This is counter-intuitive but one must remember there are more factors at play within the engine computer than simply RPMs. Looking deeper at the situation reveals that while 5th gear requires more RPMs, the ignition timing is advanced nearly 20 degrees on 6th gear plus the higher RPMs are actually in the engine's torque zone meaning it requires less air (and hence less gas based on the stoichiometric ratio) to produce the same amount of power.
When you understand your disbelief in other gods, then you will understand my disbelief in yours.
Engine designers attempt to limit this as much as possible with something called a "windage plate", but, for over-the-road cars, it's a bit of a compromise. There are height constraints on the engine, so the oil pan can be only so large, and drivers don't want to be messing with their oil on a regular basis, so efficiency loses out a little bit when they pick a "top" oil level. Try not filling you oil up to that level. Your car will run fine with the level at the "low" marker on your dipstick and you can reduce windage a bit, which should increase mileage.
You'll need to check your oil level on a regular basis, and, of course, there's no advantage in a dry-sump engine - but I don't think any current autos have those.
While I'm on reducing internal resistance, think about using a lower weight oil, that will keep viscosity at high temps. Both Mobil One and Amsoil are good at this.
The notion here is that another point of resistance in your engine is the oil in your main bearings. These are "plain" bearings, which are actually "oil wave" bearings in that the crank actually rests on a hydrostatic wave. You don't need more viscosity for this than the viscosity of the spec'ed oil at max temp. Better to get an oil that starts out pretty close to this viscosity and then stays there.
Oh - and nobody's mentioned cleaning injectors. Yes, they clog, and they don't all clog at the same rate, so you end up with some cylinders running richer than the others. Not good for max power - or efficiency. Cheap partial fix: start using Techron. Complete fix: take 'em out and send to a specialist shop (note: talk to folks about specialist shop about Techron - all the ones I've spoke with use it). If you're driving a sportscar, there'll be higher flowing injectors available. Interesting for this discussion as you can then pick up a used set of stockers for cheap, send [i]those[/i] to the shop and not be without your ride for long.
Also: has nobody actually mentioned tire pressure, and, for that matter, tire design? there are tradeoffs here, too. Lacking a pyrometer, just try running your tires a bit over spec'ed pressure. There used to be a rule about pressure increase from cold to hot, but that was for bias tires and likely doesn't apply any more. If you've got a pyrometer, you want a nice even temp increase across the treads. And you want [i]dry[/i] air. Nitrogen is best. Scuba-air is second best (it's been very dried out). Otherwise, ask whoever owns the compressor if it's been drained recently. Try a body shop or garage where they're using pneumatic tools ,as moisture's bad for the tools. (time was, body shops were best because of the care they had to take for the paint guns, but they're not run off the same compressors any more).
Look at fuel mileage ratings the next time you're replacing a suit of tires - and remember that you're trading off mileage for something else, but for folks seriously considering some of the driving tactics advocated here, this shouldn't matter much.
Ditto aerodynamics. Air dams are easy to install, do actually work, even sub 65MPH. Get a flexible one so when you hit a curb with it when parking it won't matter.
Oh - and you could just buy a higher mileage car. My wife got one of the first US Priuses. She drives it hard and has been getting 50+ MPG for years now.
For me there is a mental cost to driving real slow and getting to your destination slowly. Lives are short. Get there as quick as you can. Time is more valuable than money.
And here I thought efficiency was related to the ratio of source temperature to sink temperature, and all this seems to do is reduce source temperature.
The water is instantly flash-heated by the fuel explosion into steam for an instant expansion of 1:800 => making all your combustion engines be a partial Steam Engine by piping some H2O mist in through a vacuum tube port.
It's flash-heated, meaning that it reduces source temperature. Bad for the efficiency. And how's the steam generated from this contraption different from the steam that burning hydrocarbons produces?
Interesting thing about putting real-time steam into a running engine is that the inside of the cylinder ~spark plug electrodes and everything~ is being STEAM CLEANED as the car runs.
How again is this steam different from the steam that results from burning hydrocarbons?
The problem with driving in the highest gear is the engine is lugging at 55-65 mph, which means the fuel isn't being fully burned, which means carbon buildup in the cylinders and on the piston head. This is nothing but premature aging of the engine.
How again does drawing heat out of the combustion reaction help with making it more complete?
When the Honda Element first came out, the automatic got higher mileage than the manual. I don't know if they've changed it or not, I haven't looked recently, but they geared the manual different - although the manual had one more gear, it was geared lower than the highest auto gear. The justification was that people who were going to tow or go light off-road would be using the manuals.
Well, some of the people were sick of it, and using similar parts from Acura (which is just high end Honda, of course), they added a sixth gear... and many people went from less than 25MPG to over 30. Would it really have cost Honda that much more to add a sixth gear?
Stupid sexy Flanders.