Treehugger Homework: Drive The Speed Limit

Surely this is stupid? Or is it me?

Lets say your journey is 120km.

At 100kph, it takes you 1.2 hours to do this. You use x amount of fuel, put out y amount of emissions etc.

At 100kph, yes, you are using less fuel etc, but you will be on the road 20% longer!

Thus, you'll still be putting x amount of fuel overall, and y amount of emissions.

You break even, but you've wasted time.

Better to ask yourself if you NEED to make that journay at all?

I agreed with everything up to the last line.



High-speed accidents are caused by the difference in speed between the vehicles. So if everyone else on the road is going 70 and you're going 55, the chance that you will be involved in an accident is considerably higher.



Yeah, you're saving fuel, but what will happen if your car is totalled and all the plastic ends up in a landfill? Let's not even talk about all the petroleum products draining all over the roadway and then the landfill.



Moving with the flow of traffic is the law in most states for this reason.

Your argument makes no sense. Driving 20% slower to save 20% gas but be on the road 20% longer means a net savings of zero.

The speed limit argument has to do with the efficiency of the engine at certain speeds/gears. Back when the 55mph speed limit was introduced in the US, that was the most efficient speed for the majority of cars at the gear required to drive at highway speeds. It's since been bumped to 60-70mph most places around the country because cars engines have improved (since the '50's, but not much more). Thus, driving 20% faster isn't merely a matter of being 20% less fuel efficient (otherwise your argument would follow that driving at 45mph would be ideal -- and it's not). It's about driving at the speed that MOST cars are most efficient because of the gears in transmissions.

Driving faster in traffic is all about the "you'll eventually catch up to traffic and have to slow down anyway" aspect. If you drive quickly, you will just get to the back end of the deadlock faster and have to sit and wait like everyone else, so in that sense. Seriously, drive the speed limit because it's safer and because *it's the law* -- in the end, on average, driving faster won't get you to your destination faster unless there are no signals and no traffic between you and your destination.

"Your argument makes no sense. Driving 20% slower to save 20% gas but be on the road 20% longer means a net savings of zero."

I knew someone would say that, because it's what I initially thought. But it doesn't work that way. The 20% more gas is not per amount of time but per distance travelled. This is mostly caused by air and road resistance, but also by engine/transmission efficiency.

So lets say that at 100kph your fuel economy is 10 liters/100 km, at 120kph your fuel efficiency is around 12 liters/100 km. You travelled the same 100 kilometers in 20% more time, but saved 20% on gas/emissions.

"Moving with the flow of traffic is the law in most states for this reason."

It is also why there are more than one lane on the highway. If people in the rightward lane are going 15-20mph above the limit, I can't imagine what the people in the leftward lane are doing.

"High-speed accidents are caused by the difference in speed between the vehicles. So if everyone else on the road is going 70 and you're going 55, the chance that you will be involved in an accident is considerably higher."

Partly, yes, and of course everybody should use their judgement while driving; My post didn't say "stick to the speed limit in all situations regardless". But going faster, even if it is with the flow, is dangerous. When something happens, you have a much shorter time to react, braking distances are much longer and the impact will be much more severe. So going with the flow if it's going too fast doesn't make you safer...

It all depends on traffic density, though. Of course in very high density going with the flow is not really an option. But my post was general and I think that people can decide when to apply it to their driving.

I'll update the post.

The reason behind driving slower (the speed limit) is that wind resistance increases as a square of your speed. So it takes much more engergy to go only slightly faster etc. At highway speed most of your energy is going into pushing air out of the way.

this is a great theory, but test it in practice for yourself. drive a full tank of gas until it runs out. try it once at 75 and once at 65 and see which gets you farther. personally, it is the 75. now if i go up to 85 i do indeed notice a decrease in mileage, but not much at all. bottom line: it all depends on your car (its engine, gear ratio, etc) and how you drive it. some engines are just plain inefficient at 65

If you don't believe it just try it. When I first heard this argument a few weeks ago I decided to give it a go for one full tank and see if my mileage improved. I drive a Toyota Echo and drive on the interstate to go to work. I got 380 miles instead of 320 miles by dropping my speed down to 65 from 80 . I also quit "scooting" while at lights, you know where everyone inches up just a little while waiting? I drive a stick so I figured I was wasting gas doing that too.

Brandon,

Could you please tell us what car you have that is more fuel efficient at 75mph than at 65mph?

I'm really curious because at these speeds the biggest factor is air resistance. So while it is probably that one car would be more efficient at 75mph than another car at 65mph, I'm a bit surprised that a same car could be more efficient at 75mph than at 65mph.

To better visualize the dilemma, just imagine that instead of air, it's water the engine is pushing against... At low speed there's little resistance so it's mostly a question of engine/transmission gearing. But once you reach a certain point, it get exponentially harder to go faster, and 60mph+ certain is that point for cars AFAIK.

God damnit, you did this once before. It DOES NOT MATTER how fast you are going. What matters is the gearing and RPM of your engine. SPEED DOES NOT MATTER AT ALL GOD DAMNIT YOU HIPPIE FUCKERS.

==

Editor's note: I guess some people don't know how to respectfully disagree...

I'm gonna get shot for this but I have drag raced at a track and here's some real proof of air resistance. The car I was using got up to 71MPH over the 1st 1/8 mile but over the next 1/8th on the same run it only managed to increase it's speed by 18Mph even though the engine was closer to it's HP peak. Once you've gotten a car moving past about 40 air resistance is the main factor.

BTW The car that I race is not the car I drive to work. That car only comes out on weekends to work (tows my trailer) or play.

The relationship between speed and miles-per-gallon is especially easy to observe in hybrid cars or any car that has instantaneous and 'trip' mpg readouts. Over at greenhybrid.com in the forums, there are several conversations about which speeds are most efficient for which vehicle. In most of the discussions I've seen there, the hybrid cars being discussed get the best mileage at around 35-45 miles per hour, with mileage gradually decreasing as speed increases from there. It's possible that some cars get their best mileage at 65 mph or even higher, but I'm personally skeptical. If it is true, though, one explaination could be that (as I understand it) car engines are most efficient somewhere in the middle of their range of power outputs. It would make sense then that the more powerful the engine, the higher the speed that sweet spot is at.

this was discussed here previously and one poster claimed his cadillac's fuel efficiency computer noted better mileage at something around 75 mph. if the computer gets its info by monitoring how much fuel is being used against the distance traveled, then it would be accurate... but it could be using a preset formula that doesn't actually monitor the fuel usage.

regardless of speed or wind resistance, if your tach is high, you are burning more fuel since that is the actual guage of how hard your engine is working (if there is a head wind, your tach will show higher rpms). seems you could skip emptying tanks of gas to figure it out and make the test with identical variables... if you're rpms are lower at 75 than at 65, then you are using less fuel... right? assuming you have a tachometer...

The concept that I believe isn't being expressed is that cars like all systems have an "optimal" running speed. Most cars run better at about 60mph thus we see fuel efficiency higher for highway driving vs. city driving.

Then to further complicate the matter, if you drive above the speed limit (the speed limits by the way were reduced in the 80's I think - to encourage better fuel efficiencies) you are driving at a speed that is more inefficient.



So while you might get 15mpg city and 31mpg highway. That 31mpg estimate is likely based off of 60mph thus driving faster would be running your car's engine at a less than optimal speed. The end result is that you may get 28mpg at 70mph, and 26 mpg at 80mph and so forth.

Travis,

It is true that cars have a sweet spot. But even if that sweet spot was at 70mph-80mph (like some german cars, I suppose), it doesn't change that they still would have exponentially less air-resistance to deal with at lower speeds (60^2=3600, 70^2=4900, 80^2=6400, etc).

It is possible to make car perform *relatively* well at high speed, but unless you drive in a vacuum, physics still apply.

A good illustration is to open your window at highway speed and to put your hand outside. That's only for something like 6 square inches. Imagine for a whole car (or a whole full size SUV). That's a lot to work against...

It's not for no reason that F1 cars are like 1 feet off the ground and super slippery. Because at the kind of speeds they're going, speed^2 is the most important factor.

What's my homework if I don't have a car?

"What's my homework if I don't have a car?"

Hmm, stay carless as long as you can and forward this post/explain what's in this post to the people you know that have a car, I guess.

MGR,

You seem quite knowledgeable about wind resistance and the math applied. I have a question:

A friend of mine used to keep his windows up and locked at all times without letting any friends roll them down because he believed this increased the wind resistance and reduced his efficiency dramatically. Is this true? Would cracking a window honestly have a significant effect on MPG?

You might also want to emphasize that people should keep their car windows closed at higher speeds.

Wow. Lots of incorrent info up above. Dont think about the 'extra time' used and multiply that against the mpg. Its MILES PER GALLON, its time is not a factor in this equation.

All cars use more gas PER MILE at higher highway speeds then lower ones. But its the SUV that really suffers at higher speeds.
http://www.fueleconomy.gov/feg/images/speedVsMpg3.gif
Shows a generic speed vs mpg chart but does not say for what kind of vehicle. I use an accurate mpg reader on my mustang and have found mpg:

55 mph 35mpg
65 mph 34mpg
75 mpg 32mpg
Average city/highway 25mpg, city 20mpg or less.

You may ask how I have such high mpg on a mustang...
Its a 6cyl manual in 5'th gear with special air intake, conticontact low rolling resistance tires, acetone in gas, syn oil with prolong treatment on engine.

I have found people actually driving slower with higher gas cost. I am. Now usually going speed limit or 5 above. Used to be 15mph more then limit, got tickets about every other year.

Just don't do it in the fast lane.

We were stuck in a 2 mile backup this week behind some idiot in a Prius doing as much as 10mph UNDER the limit in the carpool lane.

I could care less if that guy got 4 more MPG that day when it was risking the safety of 100's of people stuck behind him. Not only were people packed in tight in the lane behind him, but the remaining lanes were congested with morning traffic. That made for people making risky lane change menuevers to try to get past him.

There's more to "thinking of others" than just saving gas.

The "Transportation Energy Data Book", which is put out annually by the Oak Ridge National Laboratory, has an informative section in Chapter 4 entitled "Fuel Economy by Speed". It covers empirical studies done in 1973, 1984, and 1997 on a variety of models.

http://cta.ornl.gov/data/tedb24/Edition24_Chapter04.pdf

There's a wide variety of optimum speeds for maximum fuel economy, depending on the vehicle. You can see the '97 Celica maxxing out at 25 mph, whereas most of the others fell in the 40-55 mph range. What's interesting is that though generally fuel economy dropped from 55-65 mph, the Celica actually got better mileage.

So, I think the bottom line is that the optimal steady speed is going to vary by vehicle, but generally there's a dropoff in fuel economy at a certain speed. The vehicles they tested dropped fuel economy 13-31% from 55-75 mph.

And if you haven't seen it, the EPA has a nice, pithy guide to efficient driving and other fuel efficiency tips, and they explain what gain can be expected from each action.

http://www.fueleconomy.gov/feg/driveHabits.shtml

If you have some time, I can't recommend the "Transportation Energy Data Book" enough -- it's an excellent compendium on the subject and will point you to a lot of interesting source data, much like the "Statistical Abstract of the United States" does.

All of you are talking about slowing down at highway speeds to save fuel. What about the other side? We should be going 45mph in school zones, not stopping at red lights and stop signs, and generally speeding in the city to save fuel. ;-)

In my previous car, 45 was much more fuel efficient than 55 or 65. However in the car I drive now, 65 is just as good as 55. It was a big surprise to me, because I had been used to the "55 is more fuel efficient than 65" for the past 15 years.

I'm sorry but the blatant mis understanding of simple physics is quite astounding. travelling over the same distance but faster requires more energy. It's as simple as that. Someone has mentioned F1 already, if you have watched it since they introduced refueling in the mid 90's you will know that, for example, a safety car situation means everyone drives much slower (say 75 avg mph vs 150) and in turn save something like 2 laps worth of fuel.

It's not just about air resistance but it is a major part. anyone who has done physics at about age 14 (at least here in the UK) will understand the basic principles.

Kip Kirwan,

I think that the answer to your question is in there:

http://www.treehugger.com/files/2005/06/windows_up_or_d_1.php

But the short version: Windows down at low speed, not much of a problem. Windows down at high speed: Almost like having a small parachute behind the car.

This assumption is WRONG ! I don't know how you can logically say 'All cars operate at maximum efficiency at the speedlimit.'

You want better gas mileage? Get a standard and get to fifth gear ASAP. I used to drive on back roads 35-40 mph, and got 23-26 mpg, cruising in 3rd/4th gear. Now if I pop into 5th once I get a bit over 35mph, I get between 34 and 36 mpg. Accelleration is ass in 5th, and I drop down going up hills, but the increase in mileage is worth it.

As for the highway, I cannot tell the difference in open road fuel consumption between 70->85mph, it swings between 31 and 33. HOWEVER, if I don't maintain a steady speed, this quickly drops down to 28, so if the traffic flow is heavy, I will only go 85 mph, if I can go 85 mph the whole way. If I keep running into 75mph cars and slowing down, waiting for them to pass, and getting back to 85mph, my milage will suck.

Anyway, stop these stupid blanket statements.

Brian King,

Tall gearing will help a car do less badly at high speed, but it can't magically bend the rules of physics and make air disappear. Please read other comments above for a pretty good explanation of how things work.

I think that one thing that might account for some of the misinformation is that some car's "gas mileage computer" doesn't calculate the real world gas mileage but instead works with some formula that takes into account speeds and such, but DOESN'T take into account things like air-resistance.

That would give out wrong mpg numbers and confuse people.

i don't think you're correct on that, MGR. the formula wouldn't need to account for air resistance if it used rpms against other info. all factors would be automatically taken into account because it is based on how hard the engine is working. for example, my old chrysler's trip computer would give a lower mpg when going uphill which is essentially the same as wind resistence in that there is no way for the car to 'know' about that factor.

it seems pretty simple... if you're running higher rpms, then you're using more gas. 55 mph in 4th is less efficient than 60 in 5th because your rpms drop.

hijiki,

I said that some cars' computers might be that unsophisticated. I don't say that all of them are like that.

As for RPMs, it's more complicated than that. No engine has a flat horsepower and torque curve. So depending on gear and where you are in the curves, and whether you are loaded or going uphill or downhill, gas comsumption will vary.

In short, 5000 RPM is not necessarily 2x more gas than 2500 RPM, and torque might peak at a certain point in the power band while HP will peak somewhere else, and in some situation you might need the torque while in other it's less of a factor in relation to speed.

I'm not saying it can't be done, just that it's not that simple and that many carmakers probably have fairly simple formulas that give "accurate enough" results up to certain speeds, and then they become less accurate.

Even speedmeters usually lose accuracy above a certain speed (this can be tested with a GPS that tells you your speed).

yes there are torque variables, but that's making it far more complicated than it needs to be. the best way to tell your own car's most efficient top speed in any given situation is to look at the rpms while cruising at 55mph, then go up to 65mph and look again. if it's turning higher at 65, then you are burning more gas because your torque, horsepower, terrain, wind resistance, etc aren't changing significantly. this is far more accurate than burning a tank of gas at one speed vs. a tank the following week at another speed.

hijiki,

I totally agree with your last post. 100%.

What I was saying previously (adding to what someone else had said above) is that it's possible that in some cars the unsophisticated trip-computer will tell you are that you getting better mileage at 75mph than at 65mph while it might not be true in the real world.

I completed my homework assignment yesterday. I live in Miami and was driving on I-95, at least a 5 lane highway each way. In order to go 55, the speed limit, I had to be in the very slowest lane. I'm going to keep trying to do the homework, but obviously, I'm the only one around here, because I think every single car out there passed me.

I had to put the car on cruise control (is that bad? mostly flat land here), because I found that if I was pushing the pedal, I wound up at 65.

The toughest challenge will be to get away with driving the speed limit while other people are in the car, because it definitely feels like you are creeping along...

You're right on with the idea of reducing drivng speed, but the issue is not gallons of fuel consumed per hour. The issue is air resistance, and people need to understand that.

Unfortunately, most people will not slow down voluntarily. The president must gather the political fortitude to take a leadership role on the issue of fuel conservation. He needs to take a highly active role in promoting the idea himself with a hard-driving (pun-intended), highly visible campaign to reach the American public. A few widely disseminated TV commercials would help. Bush has been a collosal failure in leadership on the issue of fuel conservation.

In conjunction with that, and very importantly, he must cause speed limits on our interstate highways to be reduced. This step would have to be accompanied by strict enforcement of speeding laws, since most people are not going to slow down voluntarily. The states must cooperate to reduce driving speeds with stricter enforcement of reduced speed limits. Without extensive enforcement, impatient and uninformed drivers will cause more accidents if they continue to speed, and will continue to consume fuel at an unnecessary and unacceptable rate. (Such cooperation would probably come about only if the Federal government twists the arms of reluctant state and local politicians by cutting off Federal highway funds--another role for leadership.) A few suspended drivers' licenses and some hefty speeding fines would have more than a salutory effect on uncooperative, uncaring drivers. We must all write to our state legislators, our representatives in Washington, the White House, and any other organization or branch of government in a position to bring drivers to consume fuel more responsibly and less selfishly.

People who argue that their cars perform better at high speeds are deluding themselves. The faster we go, the more energy it takes to overcome "drag" (air resistance). Here's an informative web site with good statistical data: http://www.fueleconomy.gov/feg/driveHabits.shtml.
P.S.: why are people still out there still burning fuel drag racing? Overactive testosterone? Don't they realize that the very heart of our economy is threatened?

Wow I've just read the whole list and am now fully convinced that the only way to reduce fuel consumption is through education not legislation
but then hey I never listened in literature classes so I guess some people found science just as dull!
More work requires more energy
Drive fast use more fuel

oh yeah while we are at it
High speed accidents are caused by cars bumping into each other, only the severity is affected by the speed differential, it ain't safer to drive fast.

And another thing
Lower city driving mpg is becuase of the starting and stopping and increased fuel used to over come the inertia of a fat lump of steel, not running at less than optimal speeds

and finally good discussion friends
slow down
relax
it might be a more pleasant driving experiance

what is the dollar/social value of that?

drive safe
happy xmas

Very nice article and comments, these gave me insight into other people's experiences and opinions.

I feel passionate about this issue, and I have been driving within the speed limit for a few weeks now.

I would like to introduce http://thespeedlimit.org: It is a place where you can discuss issues regarding speeding, the speed limit and courteous driving.

I drive a great variety of cars. I don't own, so I rent from Uncle Avis. I can say that, for sure, all cars are different. Most of the cars now, especially from GM, have MPG-meters built in. They're all different. And I usually drive with the trip computer set to it. I've tried with and without A/C, windows up and down, 70, 80, 65. Everyone of those elements makes a difference.

The last car was a PT Cruiser, which got 25 mpg on a 90 mile ride. There I was in a hurry, going about 80. So I was getting closer to 20 mph. On the way back, not so much in a hurry so that I was going about 70. 24 mpg.

My old '97 Acura CL got close to 30 mpg when she was new, but after the 45,000 mile tune up, I was getting 35 or better. Still, going about 70 mph (that's comfortable for me).

Moral of the story? I now rent based on mileage, not the car. I am a huge car geek but my four wheels now consist of a Schwinn and a LeMond. I'd rather a Cobalt with nearly 40 mpg on the highway than a G6 with 28 mpg.

Keep your tires inflated. Use the A/C sparingly. Keep your speed reasonable. Keep your car tuned up. All of these things will help keep your car forever, and keep the mileage down.

Sure it is the law. It is the people who put in too low speed limits, that needs to get removed from their elected positions. If President Eisenhower would be alive he would come and kick a couple police chiefs and judges in the nuts.

Over the years I have received a whole bunch of speeding tickets, on rural highways and interstates. I've grown sick of the cops milking money out of interstate drivers. ** I think every educated driver should read through the AASHTO Green Book. It explains how the design speed in USA is the guaranteed safe speed, for any vehichle high or low, on the tightest curve of the road. ** So when design speed is 65mph you can drive that speed continuously without breaking, provided that the weather is fair. Furthermore, if you drive a better car, or the road is straight, then you are *double safe* Unfortunately the US policy is also such that the posted speed limit is never set greater than the design speed, instead it might be even lower, for liability reasons. The result of this tripple pampering US drivers is that 95% of all sensible drivers exceed the posted speed limit more or less. The remaining 5% are just a pain in the neck, causing really dangerous overtakings. However, in bad weather, the posted speed limit, despite GOOD INTENTIONS, is way too high. The speed kills lobbyist, with the help of the local police have lobotomized the drivers, so they can no longer tell for themself what is safe or not, anymore.
http://www.fhwa.dot.gov/Environment/flex/index.htm

"There's more to "thinking of others" than just saving gas."

This is a straw man. For every driver going too slowly in the fast lane there are ten drivers speeding, passing on the right, and otherwise driving recklessly. But we are so immune to this callous behavior that the slow driver stands out.

Being scrupulously aware of the safety of ourselves, those in our car, and those in other cars will have the secondary effect of saving gas and reducing CO2 emissions because it inevitably means slower, smoother driving. Think "airline pilot" -- reserved, but highly competent and very aware -- and not "grandma".

The next time you drive, notice what other drivers are doing, and notice what you yourself are doing. Are you patient? Are your expectations of other drivers reasonable? How are the drivers around you treating others?

I am generally appalled that we (Americans) behave the way we do towards eachother while driving. In a car, the gloves are off and civil behavior is abandoned by a large part of the population. I find it bizarre.

If you no longer care about the physical safety of others, I can't imagine how you're going to be convinced that saving gas or CO2 is important.

But any thinking person can easily balance appropriate driving and get fuel savings and CO2 at the same time. There is no contradiction.