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?
My plan for improving the world's fuel consumption (Score:5, Insightful)
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.
Lower speed = Better MPG for me (Score:4, Insightful)
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.
Re:Fuel economy (Score:5, Insightful)
I've gotten lazy with this. When I know a stop is coming up, I put the car in neutral and let it coast. After a while, my clutch leg starts to hurt, so it's easier this way. People may think I'm weird, but I do it up to a mile away. The car coasts really well, so I'm usually not going any slower than I should be anyways. :)
Acceleration, not speed (Score:5, Insightful)
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%.
Re:What works: (Score:2, Insightful)
Re:Its not fuel economy, its self righteousness (Score:5, Insightful)
Re:My plan for improving the world's fuel consumpt (Score:5, Insightful)
Re:Fuel economy (Score:5, Insightful)
Holding in your clutch a lot will stuff your throw-out race. That's really bad, because that'll lead to a clutch that you can't engage or disengage. Not having a clutch when you need it is really really bad.
Engine braking is good practice. Putting aside the cleverness of modern ECUs, most footbrakes fade with use as they heat up. Yes, even disc brakes; they're a heap better than drum, but they can still overheat very rapidly.
If you're coming down a very long, steep hill and you're not engine braking, your brakes will be much less effective by the time you reach the bottom.
Brakes are a safety device, not a speed control tool. You brake only when required. Your accelerator pedal and engine braking is what you use to control your vehicle's speed. If you need to use your brakes other than to come to a stop, you're pushing your car too damned hard, or you're too damned close to the guy in front of you. Back off.
Re:Fuel economy (Score:5, Insightful)
Foolish. All engines are designed to brake.
Brakes are a safety device; you need them for keeping the car stationary, bringing it to a stop from low speeds, to prevent emergency situations from occurring, and for use in an emergency situation itself.
You save them; you don't use them. If you use your brakes as a matter of course, to control your speed, then you won't have them when you need them, due to heat build-up. No, disc brakes won't save you - they're better than drum, but they still heat up and loose effectiveness. No, ABS won't save you, either. It still has nothing to play with if your brakes have heated up.
Which would you rather: a bit of fuel economy (dubious anyway), or a large funeral bill?
Mod parent up. (Score:4, Insightful)
more time stuck in traffic (Score:5, Insightful)
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.
Re:more time stuck in traffic (Score:4, Insightful)
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?
Re:Fuel economy (Score:4, Insightful)
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.
Mod parent wrong (Score:4, Insightful)
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:BMW on fuel efficient driving (Score:5, Insightful)
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).
Highway versus city (Score:3, Insightful)
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.
Re:more time stuck in traffic (Score:2, Insightful)
Re:Fuel economy (Score:3, Insightful)
Re:Fuel economy (Score:5, Insightful)
Exactly.
If you drive well, you don't use much the breaks. In fact, the ideal ride is one you don't use the breaks at all, save for full stops.
Re:there's no such thing as price gouging (Score:3, Insightful)
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.
Re:BMW on fuel efficient driving (Score:5, Insightful)
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.
Re:Fuel economy (Score:3, Insightful)
I mean...I know gas is high, but, if you have to go these extremes...look for a career change, eh?
Re:My plan for improving the world's fuel consumpt (Score:3, Insightful)
Tachometers show us speed
Does your car only have one gear?
Re:Other helpful practices: smart braking (Score:1, Insightful)
I hope you don't do that where there is a turning lane. I have lost count of the number of times I have lost my right turn light because some numpty cruises up to the red light and blocks the entrance to the right turn lane.
Re:My best mileage is around 55 mph (Score:2, Insightful)
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.
Re:Fuel economy (Score:5, Insightful)
Three reasons this is a bad idea. One, if you suddenly have to swerve to avoid something, you might miss the edge that the power steering would provide Two, you might need to suddenly apply engine power in some emergency situations, like if you hit black ice and start skidding. Three, if you accidentally turn the key too far back, you'll engage the steering wheel lock and won't be able to steer at all.
In short: really bad idea.
Re:Fuel economy (Score:5, Insightful)
Four, if you need to stop suddenly, I hope you have strong legs. Those brakes don't work very well when your system isn't pressurized anymore.
Five, your cooling system may no longer be working, and if you are riding the edge of an overheat (considering you're driving far enough to consider killing the engine on coast) this may push you over the edge.
Re:Fuel economy (Score:5, Insightful)
Really? How many more miles does he expect to get on that engine before the repeated stress of starting it burns out the starter motor, at a minimum? Seriously, when the engine isn't running, the oil pump isn't running. When the oil pump isn't running, the oil runs down into the drain pan (especially when it's already hot). When you start a car, the cylinders are underlubricated until the oil pump gets things moving again. How many gallons of gas to you have to save to pay for a new engine, both in economic and ecological terms?
Also, keep the jerk in mind the next time you have a smoggy day. What did you think happened to all that unburned gas in the exhaust? Catalytic converters aren't magic, you know.
Re:Fuel economy (Score:2, Insightful)
Wouldnt the initial fuel injection required to restart the car exceed the potential savings anyway?
Re:Fuel economy (Score:3, Insightful)
Or wishful thinkers, yeah. Given the physics of the thing, it's pretty darn unlikely, and getting more unlikely the higher speeds are claimed as "optimal".
Wind-drag is by far the dominating force in high-speed level driving, and it goes up significantly faster than linear, typically with the speed squared.
Your engine is seriously unlikely to be so much more efficient at 2500rpm, compared to 1800 that it MORE than compensates for the extra drag at 85, rather than 60mph.
What is more likely is that often when you're driving slow there's a reason for it, curves, high traffic, hills, whatever. *those* thing will hurt your fuel-efficiency. But that's not the same thing as saying the lower speed alone hurts.
Re:Fuel economy (Score:1, Insightful)
With manual transmissions, you can just engage the clutch in a high gear and the engine starts right back up. With automatics, it's not recommended, not because of the starter, but because the pump in the transmission stops working, and the stuff that's mated with the drive wheels will still rotate, and eventually not be adequately lubricated. Manual transmissions have a more open case layout and the gears provides the needed splash lubrication.
The amount of drain down especially in a few minutes is fairly insignificant. The cylinder crosshatch retains a good bit of oil, and its not exactly thin, light bodied stuff. Also, the oil control rings and 2nd compression ring removes much of the oil that exists on the cylinder bores. it's not exactly a critical thing. Also, the oil pump would start delivering full pressure again almost instantly. It's not the only factor for cylinder bore lubrication.
What unburnt fuel? Injectors stop firing as soon as the power is cut, even though the engine may still rotate a bit longer. And that pumps only air, which is what you need to counteract unburnt fuel in the converter. And depending on your startup method, they will either deliver a bit more fuel than needed or pick up where it needs to be. But the catalytic converter will still work because it's still is hot enough to do it's work. Fact: Most of the emissions from cars nowadays comes from the first couple of miles when the engine is still cold, and the converter isn't at light-off temperatures. After that, it's immeasurable.
Re:Fuel economy (Score:3, Insightful)
Yes, but if you KNOWINGLY enter onto the Motorway with a condition on the car you KNOW could mean you break down, so deliberately driving on with low fuel, THAT is against the law. Regular car problems are unavoidable.
Re:Fuel economy (Score:4, Insightful)
Why the fuck is this modded as redundant? While it may be a case of "Yes, it's against the law, but..." it IS a safety issue. On several occasions I've had to get the hell out of the way quickly, and had I been coasting in neutral, I'd most likely be dead now.
I'd rather have a dead clutch than a dead me. Besides, with a knackered clutch, going up hills is MUCH more entertaining. Will you make it to the top? YOU JUST DON'T KNOW! It's like an extreme sport. Make it to the top of a particularly steep hill with a slipping clutch and you should really go buy a lottery ticket, as you're clearly one lucky SOB.
Re:BMW on fuel efficient driving (Score:4, Insightful)
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).
Since your "exception" is basically 90%+ of cars on the road, you are actually agreeing with the gp?
Re:Fuel economy (Score:3, Insightful)
Re:Fuel economy (Score:2, Insightful)
I don't do it, for much the same reason I don't drive whilst drunk - not being in full control over my killing machine I consider unacceptable.
Re:Fuel economy (Score:3, Insightful)
Re:Fuel economy (Score:3, Insightful)
> 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 account and it's still appauling)
Re:Fuel economy (Score:3, Insightful)
Not just weird; dangerous for reasons other posters have cited.
Hypermilers are starting to get really annoying. I was behind a person the other day who refused to go more than 30 in a 40mph zone on a road where one cannot pass. The line of cars full of unhappy people behind me got to be very long just in the short 2 mile trip to Home Depot. If that were not bad enough, she took a good half mile just to get to 30.
Driving with an eye towards fuel economy is all well and good. I pay attention to my realtime MPG readings when I'm on the highway. That being said, if one is so concerned about milage that one is going to drive in a way that impedes or endagers others then one should just stay home or take other means of transportation. If you can't afford the gas then don't buy it. If you think you're doing some great altruistic good by reducing your emissions, please sell your car and get a bike.
Mental Cost To Getting Their Slowly (Score:2, Insightful)
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.
Re:more time stuck in traffic (Score:3, Insightful)
Re:Mod parent wrong (Score:3, Insightful)