Fuel economy myths

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Ted Mittelstaedt wrote:


*NEXT* you're going to tell us that an alternator is harder to spin and therefore takes more power from an engine if you turn more electrical things on in your car!! :)
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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A former co-worker a few years ago came up with an idea that he claimed was increasing his gas mileage.
He created a water decomposition chamber out of a length of ABS and connected it to his alternator, and a tube from it to his intake manifold. He regulated the power going to the decomposer with a big carbon rheostat he found from something.
When he showed it to me I just had to smile and nod. This guy was (and still is) a computer service tech.
600 BC Greek named Thales was the first to make recordings with respect to electricity, documenting the rubbing of amber. Electricity regarded by most as black magic
1750 - Benj Franklin proves lightning contains electricity. Electricity still regarded by most as black magic
1831 Faraday discovers electromagnetic induction. Electricity still regarded by most as black magic
1904 Fleming invented the vacuum tube. Electricity still regarded by most as black magic
2007 Modern society, global communications, international diplomacy and geopolitics are utterly dependent on electricity. Electricity still regarded by most as.....
Ted
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The electricity to charge those batteries is going to come from .....?
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wrote:

Ah, that's a different topic. That question can be asked about anything that uses some kind of power.
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Joe wrote:

I think he's getting at how fast you can get the required energy transferred into the battery (without overloading your supply system and without destroying your battery with waste heat).
You can't get infinite power in zero seconds from a practical source. Look at Ted's post with quantification of the problem (i.e., 12,000 watts for 10 minutes). Actually Ted's calculations are optimistic. Whatever the inefficiencies of the charging process ('x' watts of heat generated in the supply lines and in the battery for every 'y' watts of useable charge) would only increase the watts required from the source. IOW - if the charging process is 70% efficient, that 12,000 watts sustained pull becomes over 17,000 watts.
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:

Awwww Bill, I was just -asssssuming- that the wiring would be with that room temperature superconductor wiring from Ringworld! ;-)
Ted
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wrote:

It can go both ways, but I took it as questioning the source of power for recharging. It's a valid question, but it certainly covers a wider topic than only vehicles.

Granted. But the average American doesn't give two dumps about any of that. He/she simply wants to pull in to a refueling station, spend no more than 10 minutes there, then take off for another 300 miles and do it again when the needle is hovering right above "E".
People's expectations regarding the ultra-convenience of refueling today's internal combustion engines will be nearly impossible to change.
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slowing down to 55 will do the most to conserve fuel i just read a study about mileage and cars,suv's are at the most efficient at 55mph regardless of what brand they are. between 30 mph and 50 the cars consume more as well as over 60 to 90 mph seems 55 is the sweet spot so it's no mistake that mileage is rated at that speed detroit will never become competitive until the million dollar bonuses stop for the executives and that money is reinvested back into the companies
Ed wrote:

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I've found that to be true on my own cars that have the instant readout for mileage. Repeated runs over the same piece of road at different speeds on my way to work.
Problem is, you can't do it voluntarily by yourself. It is downright dangerous to drive at 55 while everyone else is doing 70 or above. Nor can I drive a car at 55 on the open road when both the car and highway are built to sustain much higher speeds.
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I've found it to be true in real life measurements, but only in the strictest definition of the word true. In all of my vehicles, the age old adages about easy starts, easy stops, etc. have certainly been born out, but the difference in the fuel mileage between driving at 70 instead of 55, or jumping on it a little bit instead of grandfathering it off the line, was inconsequential. In order to really impact mileage I find that I have to put my foot into it completely, all the while - through the entire tank of gas. Not at all practical for most driving. Otherwise, the difference in mileage over a thankful is just not that significant.
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Since I don't drive at 55 I cannot factor that into my daily life. I can, however, compare the grandfatherly driving versus putting the foot into it once in a while, using my remote starter to hear up the car when cold or start the AC when very hot. While I don't "speed", I often do like to accelerate up the entrance ramp to the highway and be doing 70 at the top. The difference between the two types of driving cost me about 38 gallons a year over 23000 miles.
I can hear some of you saying "not big deal, enjoy driving" while others are screaming that "a family in Ethiopia can plow their fields with what you wasted". Judge as you please but I'm not going to change. Another zinger, my total cash outlay for gas last year was probably $200.
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"Edwin Pawlowski" wrote:
I can compare the grandfatherly driving versus putting the foot into it once in a while. The difference between the two types of driving costs me about 38 gallons a year over 23000 miles. Another zinger, my total outlay for gas last year was about $200. ______________________________________________________
At $3 a gallon, $200 will buy 66.67 gallons of gas. And 66.67 gallons to go 23,000 miles means the car is getting 344.98 mpg.
Driving more conservatively will save the lost 38 gallons, using only 66.67-38 = 28.67 gallons. This will result in a gas mileage of 23,000/28.67 = 802.23 mpg.
That's what I call good gas mileage. Congratulations.
Rodan.
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I said that was MY total cash outlay. I do buy gas once or twice a week. :)
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The instant readout method is a bit questionable. My wife's Sebring has it and I've watched it, but you can get a much better comparison of mpg at different speeds by viewing the mileage over a reasonable distance.
Different cars designs vary as well. My wife's 2001 2.7L Sebring drops it's mileage off faster at higher speeds than my 3.3L Concorde. Both get the same mileage at 60mph, but the 3.3L Concorde is better than the Sebring at 70mph. I've also noticed this with a few rental 2.7L Intrepids.
I keep track of all our mileage so I have lots of real data on actual trips as well as urban mileage when the Sebring is slightly better than the Concorde. Since I don't travel steady highway speeds at lower than 60mph, I have no lower speed comparisons. The figures I have are at 60, 65 and 70 mph.
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wrote:

It may be questionable as to the reading versus actual if you used test instruments, but it is consistant. I took the readings over the same stretch of road over many days. Each speed was compared in the same place for the same distance. Rounding may also be a factor, but again, it is a consistant comparison. If, at 70 I get 25 mpg and at 60 in the same place for the same distance in the same direction, I get a reading of 27 mpg, I know for sure that I'm getting better mileage, be it 1.9 or 2.1 before rounding.
I have two cars that give me the milage over the tank of gas and one is consistanly 1 mpg over what I calculate at fill-up time, the other is consitantly over 0.5 over calculated. What is important is knowing that and when I see this tank was 26.2 and the last tank was 23.5 I truly did have a 2.7 difference.
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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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