Fuel economy myths

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NOT true of all vehicles. I have driven vehicles that were more efficient at 70 than 55 (1975 Celica GT) It depends a lot on the gearing and the co-efficient of drag.
My current TransSport seams to be best aroung 65 or so (115 - 120Kph). Extra weight is a killer too. I leave the rear seats at home most of the time as I don't need them. SHould leave the middle ones too and half the crap I always carry with me.
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phil wrote:

Which is the cause and which is the effect? Are official mileage figures done at 55 mph because that is a fixed law of physics that that is the optimum (fuel mileage) speed for a wheeled vehicle. *OR* is 55 mph optimized by the manufacturers for fuel mileage because that is the speed at which the law arbitrarily says they have to be measuered at?
I know all about the square laws of wind resistance and all that, but I have a hard time believing that 55 mph is the optimum speed by a fixed law of physics and not an arbitrary design parameter. Things like this get ingrained in the culture to the point that it starts being treated as if it were a fixed natural law instead of some arbitrary phenomenon created by legislation.
I submit that if the mileage measurements were required to be done at, say, 65 mph, lo and behold, within two vehicle design generations you'd find studies that proved that 65 mph was the magical optimum fuel mileage speed. I'm not saying let's change the official measurement speed to 65 mph, but let's be careful to not make the "55 mph is the optimum fuel economy speed" an irrefutable law of physics.
Question: Would a study of all cars designed for use on the Autobahn also show that opitimum fuel mileage speed is 55 mph?
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
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I don't know enough to dispute or confirm that.
When I was buying a Mack truck years ago, the salesman had figures on power needed for various loads. At 60 mph, you needed more power to overcome wind resistance than for rolling resistance or load. It would take some analysis by an engineer to determine the if the 55 sweet spot is true or not. In the case of final drive ratio, there has to be some compromise between highway driving versus rural driving. City driving ratio selection is easily done with the transmission lower gears. My 5 speed does not shift to the highest gear until 40. Maybe a 6 or more speed would shift at 50 or higher to gain better ratios at 70.
My guess is that gearing can be better optimized at various speeds, but at some cost.
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Actually Bill I think that 55mph really is the "sweet spot" for a NON-wind optimized vehicle.
I ride a motorcycle quite frequently that has NO faring (Honda CB750 if anyone cares) with an upright riding posture. (it's the most comfortable)
I can feel force on my arms increase as speed increases. However, it is NOT linear. Up until about 55 you can barely feel it. The faster I go beyond this the force really does build up very quickly, even going an additional 5 Mph. Normally I cruise at 65-70. Other bikers in the M/C forums have noted the same thing.
I suspect the issue is the air shearability and density. I do NOT believe that this holds true for modern vehicles since they are optimized in a wind tunnel. But I am strongly suspecting that back in the 70's when they came up with this figure they got it by using average non-wind-optimized vehicles of the time, driven on a test track with controlled conditions at different speeds - since at the time the software for advanced wind tunnel modeling didn't exist.
I still suspect it is true for semi-trucks since most of them are built like bricks.
Ted
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Not entirely. You don't see cabover trucks anymore. http://www.truckpaper.com/listings/detail.aspx?OHID 24449&guid>6D1E5ECA9548F2A0479F53CF40039B
And a lot of the highway trucks at least have dams on them so the wind is not hitting the trailer right in the face... http://www.truckpaper.com/listings/detail.aspx?OHID 24796&guid>6D1E5ECA9548F2A0479F53CF40039B
And the new columbia's are designed for even more aerodynamics, but they are not really getting the fuel economy they were intended to. http://www.truckpaper.com/listings/detail.aspx?OHID 38420&guid>6D1E5ECA9548F2A0479F53CF40039B
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http://www.truckpaper.com/listings/detail.aspx?OHID 24449&guid>6D1E5ECA9548F2A0479F53CF40039B
In Europe, that is all you see. Space limitations prevent large trucks like we have. No 53' trailers.
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Edwin Pawlowski wrote:

Yes that is right, i did mean to mention that. many of them only have 1 axle on the rear as well.
Those little streets are so small you wouldn't want anything else. Trucks and trailers look so small there in comparison to what we have rolling down the highway.
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Ted Mittelstaedt wrote:

I suspect that it has to do with other things, such as there's some threshold that you start approaching some force threshold beyond which your arms have trouble resisting on a sustained basis with the force increasing by the square of the speed (I think) - to the human mind that threshold appears as some magic switch point. To someone with slightly stronger arms, the square law increase in wind force is going to make that threshold appear at some higher but approximately the same speed, so when the two people talk, they're going to agree that at around 55 mph, things "really" change.
Not sure of the validity of comparing an arm to a car body anyway. There's no question that some cars are more efficient at speeds higher than 55 - as posted by some here. I think you have fallen into the exact trap I was warning about of making a law of physics about the so-called 55 mph sweet spot. Wind resistance felt on a human arm on a motorcycle is not very convincing to me. Watching fuel mileage readouts at different speeds in different (varying degrees of streamlining and gearing) vehicles is more convincing.
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
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wrote:

I suspect it's all in his head. Unlike in a cage, you notice everything more as you go faster on a bike.

Yep.

It actually helps to prove your point. If you're riding at 55 into a 20 mph wind, the "feeling" should be different than if you're riding WITH said 20mph wind.
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Gary L. Burnore wrote:

True.
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
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Then bikes should all go 110Mph so the riders can notice everything the most. You sound like a damn Harley rider with one of those full body farings. Yick.
Ted
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No, Bill, I'm not trying to make up a law of physics here. I'm made an observation that there IS a sweet spot. I'm not saying the sweet spot exists because of an inherent law of physics. But, it does exist.
Note also that if you look into this, there are two drag calculations - linear drag and quadratic drag. The quadratic drag is the doubling of speed force quadruples which people have been throwing around. Linear drag is force of drag is proportional to velocity. The linear drag formula is used for low velocity, the quadratic is used for high velocity. For a vehicle accellerating from 0, it would fall under the linear formula until reaching a certain speed then fall under the quadratic formula.
Beyond that, not being a mathmetician, I'll let others speculate. But, I observed a sweet spot, the EPA observed a sweet spot when they were studying mileages back in the 70's. It exists.
I suspect the sweet spot in mileage is there simply because any given type of car engine has a sweet spot on the power curve - there is a section where with a given engine when you take a certain amount of power out of it, the engine runs most efficiently. Thus, given any vehicle you can measure it's drag and as speed increases see how drag increases - and at the point at which the energy to move the vehicle through the air is equal to the sweet spot on the power curve, well there you have your most fuel efficient speed to run the car.
In the early 70's it probably came out at 55 because most cars were V8's running low RPMs. No guessing in the higher rev 4 bangers that are most common what it would be nowadays. Might even be 90Mph.

Yes, but when talking governmental regulations we have to look at the average.

Then, answer this - at what speed does a car stop obeying the linear drag equation and start obeying the quadratic drag equation?
Ted
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Ted Mittelstaedt wrote:

Good question - I will research that. I bet it depends on shape - I don't know that for a fact, but I suspect it's true. Again - look at airplanes that travel at quite a bit more than 55 mph and don't hit any almost impenatrable (sp?) speed wall without additional laws of physics clearly having to be taken into account (speed of sound notwithstanding).
But whatever the answer is, there are two things staring you in the face: (1) Your own words: "I suspect the sweet spot in mileage is there simply because any given type of car engine has a sweet spot on the power curve - there is a section where with a given engine when you take a certain amount of power out of it, the engine runs most efficiently. Thus, given any vehicle you can measure it's drag and as speed increases see how drag increases - and at the point at which the energy to move the vehicle through the air is equal to the sweet spot on the power curve, well there you have your most fuel efficient speed to run the car."
Neither the engine or gearing are fixed, and the body slip-thru-air properties are also design dependent.
(2) Different cars clearly show different sweet spots as proven by instantaneous readouts (which though maybe not absolutely accurate are monotonic - i.e., relative readings can be trusted as far as "more" and "less").
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
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On Sun, 7 Oct 2007 23:56:50 -0700, "Ted Mittelstaedt"

Many things come into the equation. Coefficient od aerodynamic drag will be one of the primary ones. A vehicle with a CD of .27 will be significantly more efficient, and see a slower decay in efficiency with speed, than one with a CD of .5.
The second major contributor will be the power/torque curve of the engine. An engine produces the most HP minutes per lb of fuel when running at it's maximum torque RPM. If the torque curve is nice and fat (not peaky) and the gear ratios are correct, the car can run with the engine in it's sweet spot over a wider range of speeds, and the overlap of the engine efficiency sweetspot and the aerodynamic sweetspot can make the vehicle actually MORE efficient slightly above the aerodynamic sweet spot than at it. Obviously, the POTENTIAL for better mileage exists - but not with that particular combination.
Secondary to the primary considerations are weight issues and the related inertia. The lighter the car the better. The lower the rotational inertia of the driveline the better. The lower the friction in the driveline the better. The lower the rolling resistance of the tires, the better. The less brake drag the better. Most of the secondary issues are less speed sensitive tan the 2 primary issues, and of coarse, the "weight of the right foot" has to be factored in as well!!!
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<clare at snyder.on.ca> wrote in message wrote:

I just bought a Buick with a 3.5L engine and 6-speed transmission. Where an I find a torque vs rpm curve for this engine? Previous cars I've owned sometimes had the curves in the literature. All things being equal, it would be nice to cruise at the peak torque.
Dave C.
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the cars on the autobahn would also see the optimum efficient operation at 55 mph according to what i was reading like you said the law of physics applying here the magic number of 55 is used because of the optimum efficient operation and it was studied long and hard by the epa and gm before that figure was picked as a standard setting the speed limit to 55 would also create a gasoline inventory excess that would back up the refineries so much so that the winter and summer blend refining schedule would interfere with the season blending, they now have to use by epa law. it would effectively shut them down and they would have to give gas away to make room for the next blend coming in that is one reason why we see gas prices drop before summer and before winter so they can make room for the new blends i have a customer who works for marathon oil a higher up exec. and those are his words to me as he hands me a gas card as a tip instead of cash i drive a rt dalota and use the eway and i drive 55 to help my cut fuel bill and see folks driving by me so i am not stuck in those wolf paks of cars so i get less stone chips too
Bill Putney wrote:

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philthy wrote:

You know it makes no sense at all. Some cars are absolutely more efficient at higher speeds than 55 mph. What that says is that the optimum speed can be designed into the vehicle by body design and gearing, not to mention engine design (cams, intake tuning, etc.) and ignition tuning curves.
Some cars absolutely get better mileage at higher speeds than others. That's the starting point for my argument (that optimum speed for fuel mileage can be designed in and that 55 mph is not dictated by the laws of physics as the inherent optimum speed).

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It really doesn`t matter how slippery the body (unless someone comes up with some magic to drop below appx 3.0 which is the current norm, as soon as you go fast enough to creat a measureable wind resistance from that point ultimate efficiency will drop off. No amount of gearing will increase it at that point, unless you drop weight or increase efficieny of the power plant or drive train losses. If you vechiel is getting better milage at above 55 than at or below it, it is comprimised in one other department and could do better. The laws of physics can not be changed only the paramaters varied. KB
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Kevin Bottorff wrote:

Makes one wonder how airplanes can go above 200 mph efficiently without them seeming like they are moving thru glue then doesn't it.
I question your use of the term "measureable wind resistance". Depending on your instrumentation, that point could be practically anywhere, including a fraction of a mph. Your statement is meaningless.
But - yes - everything is a tradeoff. It's all *incremental* and continuous variations of all the parameters involved (until maybe you hit the sound barrier). Not some almost brick wall function as some seem to be implying. What you say about compromise is exactly correct. That's always true. Where does one draw the line. Does this mean that 45 mph is inherently more efficient than 55 mph, that 35 mph is inherently more efficient than 45 mph, that 20 mph is inherently more efficient than 35 mph, that 2 mph is inherently more efficient than 3 mph...?
It all boils down to what are we willing to accept in the various areas of choices. Then decide to optimize according to the arbitrary priorities. Something magic about 55 mph? or 100 mph? or 35 mph? or 2 mph? I don't think so.
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
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That is one reason jets climb to 30,000 feet or more, to reduce wind resistance.

The magic may not be the laws of physics as much as it is a good place to compromise.
National Geographic Magazine has a chart showing the average mpg of eight (un-named)cars and light trucks. The curve is relatively flat from about 35 to 55 mph and drops quickly from there. It is just a mention in an article about Germany wanting to reduce carbon emissions by reducing the Autobahn speed limit so details are lacking.
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Ed
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