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 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?
Re:Fuel economy (Score:1, Interesting)
And don't engine brake because that is poor form and is retarded. Use your damn brakes.
It's about acceleration (Score:5, Interesting)
Re:Fuel economy (Score:5, Interesting)
I would always just leave the clutch in rather than put it in neutral in case you have to get out of trouble.
I remember once, on the M27 in England, a friend and I were driving to Portsmouth. We'd be put just over a pounds worth of petrol in. Gauge didn't even register empty. It was below that. There's a long downhill bit heading down into the city. He put the clutch in and we coasted several miles at least. (This was in a really old Saab.) Little disconcerting, especially when venturing onto a Motorway in the knowledge that your car may break down is an offense.
Re:Fuel economy (Score:3, Interesting)
You're proud of 30mpg?
I can commute about 40 miles to work in the rush hour traffic and get over 50mpg without even trying.
Outside of rush hour, a leisurely cruise back along the same route easily gets 60mpg. Best I've ever achieved was 77mpg, but that was actually quite a lot of work. It's not constant speed that you want for maximum mpg, and although using a cruise control gives a reasonable zeroth order approximation, you can do better. Of course, if the road is 200 miles long, with no variation in incline or corners, and no traffic causing you to slow down unnecessarily, then cruise control will equate to the best mpg you can get :-)
If I drive like a nutter, I expect to average around the 26mpg.
Maybe you guys need some European cars? You'd certainly not be satisfied with 30mpg at our current fuel prices ($2/litre)....
Don't be aggressive (Score:3, Interesting)
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.
Re:Fuel economy (Score:2, Interesting)
May someone tell me if you are supposed to switch over to N on an automatic if you stand still at say red lights or such for a short while or should you just release the gas and break?
Re:Fuel economy (Score:2, Interesting)
...Not having a clutch when you need it is really really bad...
True, but I've had to drive without a working clutch (always engaged) for at least a week on two occasions. Both were the result of broken clutch cables. Most likely the cables broke from excess use of the clutch pedal. The routine was the same both times;
Then there was the time that my starter fell off (probably from starting the engine in gear too many times)-; and I had to always remember to park facing down hill....
I'm so glad I make enough money to afford a reliable car now;-)
As far as engine breaking goes I agree 100% it's the right thing to do in most cases. And the people who try to stay at full speed right up to the last moment then apply the breaks are the retards!
Re:more time stuck in traffic (Score:5, Interesting)
You have the wrong feeling. Driving the speed limit is a good idea. Driving far below the speed limit (as the submitter specified people going 30 MPH in a 45 MPH zone) is risky and detrimental to good traffic flow and traffic safety as a whole.
The driver going far below the speed limit is likely to incite someone who is less patient to recklessly try to pass them, and that slow driver is responsible for helping to create that situation. To say otherwise is to tell someone poking a bee hive with a stick is not responsible for the stings they receive - that it's only the bees that should be blamed.
In my eyes, anyone who intentionally drives in ways that are counter to how a traffic system has been designed and implemented is putting the rest of us at risk... that means excessive speeding as well as excessive slow driving.
I've studied traffic a lot over the years and what I do know is that it only takes a few drivers driving in selfish ways to really screw things for everyone else. The guy racing ahead on the right to merge at the last minute tends to be the same guy bitching when someone else cuts them off.
And to say "I'm getting better gas mileage, everyone and everything else be damned" is just more of the kind of thinking that leads to all of us getting screwed.
Re:Fuel economy (Score:2, Interesting)
He might have one of the Audi/VW DSG systems which basically uses a double clutch to automatically control a manual gear box. My dad's couple year old A3 has this
Re:Mod parent wrong (Score:4, Interesting)
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.
Re:Fuel economy (Score:2, Interesting)
Re:Fuel economy (Score:3, Interesting)
Best practices (Score:3, Interesting)
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.
Re:Fuel economy (Score:2, Interesting)
And as an ex-defensive driving teacher, I can tell you that shifting into neutral while driving can be not only dangerous, but is against the law in most US states.
Re:Fuel economy (Score:5, Interesting)
Coasting may be illegal in most US states, but so is travelling five miles an hour over the speed limit. Fortunately, most cops aren't about to pull me over for either (though I do stick to the limit in school zones, for obvious reasons), so I'm not liable to not coast simply because it's illegal.
My reason for not coasting? From what I understand, when the engine's turning above ~1,000 RPM, the throttle's at "idle" (no pressure on the pedal), and the transmission's in gear, then the fuel injectors shut off. For everyone but the parent poster, that means it's not burning gas, and thus raising the mileage. Whenever I might use the coasting technique, it's probably better to simply leave it in gear, let the injectors shut off, save gas, and save my brakes (without worrying about overheating them, too).
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
Hope you're not driving in Seattle... (Score:2, Interesting)
Re:Fuel economy (Score:5, Interesting)
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
This is common on a lot of cars. I drive a 2006 Impreza STI and I can absolutely confirm that the injectors are off if you are engine braking. It's the only time EGT drops all the way to zero, even at idle EGT is still around 700*. My 2003 Mustang Cobra did exactly the same thing with the stock tune, verified in a similar way. I'm pretty sure most cars do this -- at least the manual transmission ones. If you leave the injectors firing at all when engine breaking, it causes popping & gurgling afterfires that sound like an old pickup truck.
Re:more time stuck in traffic (Score:1, Interesting)
No, there are studies done by the California Highway Patrol and others that list driving slow as a major cause of accidents and far more dangerous than driving faster than traffic. It was ranking causes of accidents driving slow was like number 4 or 5 while driving faster than traffic was somewhere around 18 in the list.
Besides being dangerous it is also very rude and selfish
Re:Fuel economy (Score:2, Interesting)
Have you done a comparison against whatever speed your car does in 5th gear at 2000rpm?
Cheers,
Michael
The Physics of it all (Score:2, Interesting)
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).
TransAms (Score:3, Interesting)
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.]
Re:Fuel economy (Score:3, Interesting)
How exactly is it putting "extra stress" on it? You're taking in, compressing, and exhausting air, as opposed to taking in and compressing air, shooting in some gas, blowing the lot up, then exhausting the remains.
Then again, IANAMechanic.
Fiat punto stationary in fifth 2.5l/100km (Score:3, Interesting)
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.
Re:Fuel economy (Score:1, Interesting)
On the flip side, brake pads are cheaper than gearboxes.
Re:Steam Plus Gasoline Engine Systems (Score:3, Interesting)
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?
Re:more time stuck in traffic (Score:3, Interesting)
True, which is why I said 'most of the time'. (Some places even intentionally set traffic lights to stop you, especially at night.)
But if you don't know the timing, the best guess is that they are timed to the speed limit. That's the general default, unless there is some reason to set them otherwise. (Like it being a bad timing for the cross-road that is busier, etc.)
(And it's a personal pet-peeve of mine at the moment: The road I drive to work every morning is timed to the speed limit. There is nearly always someone driving 5MPH under the speed limit. So we hit every light. If I'm not stuck behind someone going 5MPH under, I can breeze through without slowing down. If I am, it doubles the amount of time I'm on that road.)
Re:It most likely varies from car to car (Score:4, Interesting)
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?