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Old 08-16-2015, 11:36 AM   #29
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doubling your speed will quadruple the wind resistance.


Yes, that's true, but the speed/mileage spin doctors lie to you by not mentioning that as you double your speed you actually go farther in the same time, so your fuel consumption goes up linearly, not as a square. Fuel consumption is measured in gallons/minute, but mileage is miles/gallon. It's deliberately confusing. The EPA tells a lot of lies, with good intentions, of course.


Also, air resistance is far from the only drag on your coach.


Still, that's good mileage on a pretty big coach.


Tom
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Old 08-16-2015, 11:58 AM   #30
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Originally Posted by TLGPE View Post
[COLOR=darkred]Yes, that's true, but the speed/mileage spin doctors lie to you by not mentioning that as you double your speed you actually go farther in the same time, so your fuel consumption goes up linearly, not as a square. Fuel consumption is measured in gallons/minute, but mileage is miles/gallon. It's deliberately confusing. The EPA tells a lot of lies, with good intentions, of course.
Hi Tom,

You got me thinking. However; I'm not exactly sure what your point is.

If we use some arbitrary numbers such as a 1000 mile trip, $3/gallon fuel, 10 MPG at 60 MPH, and 9 MPG at 65 MPH we find that:

Trip A (60 MPH) requires 16.66 hours, 100 gallons of fuel, and $300.00.

Trip B (65 MPH) requires 15.38 hours, 111 gallons of fuel, and $333.00.

If that is reasonably accurate, are you saying that 65 MPH is a less expensive trip because the (approx.) one hour and twelve minutes is worth more than the fuel savings ($33) or something else?

Thanks!
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Old 08-16-2015, 12:19 PM   #31
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No, JD, nothing like that. Just soap boxing about misleading statistics. Back in the seventies, everyone was saying that reducing your speed by ten percent would increase your mileage by twenty percent. That's wrong on several counts, the first being that the math is just wrong.


The face is, that real measured studies show that in the normal range of highway speeds, mileage goes down at a rate of about half of the speed increase. In other words, ten percent increase in speed causes about five percent reduction in fuel mileage.


So, I usually drive at about 60 to 63 mph. Slower is cheaper, but takes longer and blocks traffic. Sometimes we are in a hurry and go 65 to 68, buy usually not.


Anyway, just chatting on a Sunday morning.....


Tom
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Old 08-16-2015, 12:33 PM   #32
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Ahh yes - that does sound familiar. I understand now. Thanks Tom.
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Old 08-16-2015, 05:51 PM   #33
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Wind resistance increases with the SQUARE of the velocity. Yes, doubling your speed will quadruple the wind resistance.
We're talking two different things. The power required to drive a vehicle is the cube of velocity. What Is Air Resistance
Power requirement is what governs fuel mileage.
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Old 08-16-2015, 08:11 PM   #34
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Wind resistance increases with the SQUARE of the velocity. Yes, doubling your speed will quadruple the wind resistance.
Here is the actual formula:
Power

The power required to overcome the aerodynamic drag is given by:
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.
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Old 08-17-2015, 08:50 AM   #35
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tire pressure setting is a tricky thing...too many variables...i wonder how other rvers weigh their rvs - . with full load (for example, fuel 100 gal, fresh water 100 gal, maybe hslf tank of grey and black...)? there will be a big difference between yes and no.

but next time i'll weigh and see.

happy trails to you and all!!
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I weighed the coach with full fuel (150 gal), full fresh water(90 gal), full refrigerator, pantry, and basement freezer. All our clothes, my tools and anything that will fit in the basement. Then, I did a six position weigh with individual scales. That is the only way to know what the tires are carrying. Once I had the weights, I looked at a load chart for the heaviest tire on an axle and added 5 psi to that number. Then on a 65* morning, I adjusted the pressure on all of the tires at 5 psi higher than the chart called for. That was over a year ago and I have not had to add air to any of them since. It works great. As example, the fronts called for 110 psi and I put them at 115 psi. Over the course of the past year we have been at altitudes from 1000' to 11,000' and temps from 40* up to 100*. The cold starting pressure ranged from 110 psi up to 120 psi depending on temp and altitude. Above the minimum and just at the max pressure for the tires.
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Old 08-17-2015, 10:18 AM   #36
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I weighed the coach with full fuel (150 gal), full fresh water(90 gal), full refrigerator, pantry, and basement freezer. All our clothes, my tools and anything that will fit in the basement. Then, I did a six position weigh with individual scales. That is the only way to know what the tires are carrying. Once I had the weights, I looked at a load chart for the heaviest tire on an axle and added 5 psi to that number. Then on a 65* morning, I adjusted the pressure on all of the tires at 5 psi higher than the chart called for. That was over a year ago and I have not had to add air to any of them since. It works great. As example, the fronts called for 110 psi and I put them at 115 psi. Over the course of the past year we have been at altitudes from 1000' to 11,000' and temps from 40* up to 100*. The cold starting pressure ranged from 110 psi up to 120 psi depending on temp and altitude. Above the minimum and just at the max pressure for the tires.
this sounds all considered, awesome. i'll this when back to us. enjoy your rving.
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