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Old 07-09-2013, 01:05 PM   #21
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Originally Posted by w4drr View Post
It has been my experience through the years, both claimed and actual mileage while towing can be reduced to 2 simple facts:
Fact #1: The BTU's expended to accelerate the mass of a TT or fiver and overcome wind resistance and other friction losses is pretty much governed by the laws of physics. It always seems to average around 8-10 MPG for gas, and 10-13 MPG for diesel. Diesel always has the advantage primarily due to the fact there are more BTU's per gallon of diesel than there are in a gallon of gasoline. Diesel also has the compression ratio advantage.
Fact #2: People tend to exaggerate and/or lie about the mileage they really get.

Bob
X2 to both. I tow my V-Cross 9280 lbs at 68mph and almost consistently at 10mpg. My previous 5er was 14K lbs and I got 9 mpg. With mod's I think I could bump that up by maybe 1mpg. Can't fight physics. If I tow at 60 I could do better, but I am to impatient.
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Old 07-09-2013, 11:02 PM   #22
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.........but then again, if you have a flux capacitor available, the sky's the limit!
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Old 07-09-2013, 11:31 PM   #23
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I have wracked my pilot brain for a few years now over the "why" that regardless of towed weight, the mileage is fairly constant for gassers and diesels.

After much thought and several articles on the topic, I have come to believe the largest factor is not the weight of the camper but frontal area and speed.

What I have observed here:

1) Travel trailers seem to get better fuel economy (as a class) than 5th wheels.

2) Diesels get better fuel economy than gassers (but cost way more).

3) The larger the frontal area of the combination; the more wind drag; thus higher throttle for the same weight.

The higher the impact speed on the frontal area; the higher the drag and the lower the fuel economy.

Example:

Take a sheet of plywood and holding it flat in front of you and walk briskly.
Now stand in the bed of a pickup truck and see how long you can keep from being blown off the truck as it accelerates.

Remember also that the HEADWIND component of the relative wind must be added to the speed of the truck to determine the drag factor. If there is a TAILWIND, you SUBTRACT the wind speed from the speed of the truck.

That is why some folks can honestly say they get unbelievable mileage, but only if they do the test on a certain leg with a following wind.

With 20 MPH of headwind, driving at 65 has the drag effect of driving 85 MPH.
With 20 MPH of tailwind, it would have the drag of driving 45 MPH.
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Old 07-09-2013, 11:49 PM   #24
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Originally Posted by herk7769 View Post
With 20 MPH of headwind, driving at 65 has the drag effect of driving 85 MPH.
With 20 MPH of tailwind, it would have the drag of driving 45 MPH.
Great explanation, Herk.

And I claim 20mpg in my Class C (Only when talking with the guy who claims 15mpg in his 5er!
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Old 07-10-2013, 07:14 AM   #25
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Herk, that is exactly right. The biggest factor is wind resistance pulling that big "sail" behind you. That is why mileage is fairly constant, regardless of weight. That is also why we downsized to a pop-up. We have reduced the frontal area, and voila! Our mileage is now much better. We are seeing 13-14 MPG, as opposed to the 8-10 we were seeing pulling our TT.

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Old 07-10-2013, 08:05 AM   #26
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Herk, that is exactly right. The biggest factor is wind resistance pulling that big "sail" behind you. That is why mileage is fairly constant, regardless of weight. That is also why we downsized to a pop-up. We have reduced the frontal area, and voila! Our mileage is now much better. We are seeing 13-14 MPG, as opposed to the 8-10 we were seeing pulling our TT.

Bob
That makes sense, if you didn't improve that much without the wind resistance, I would say you would have a real problem......
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Old 07-10-2013, 08:54 AM   #27
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I also have to add that when calculating the effect of drag, the effect increases with the square of the velocity. A 10 MPH increase will have 100 times the increase in drag.

That is why a 5 MPH deduction in cruise speed will increase my MPG a full MPG at 60 vs 65. Don't even want to tell you what my MPG drops to at 70 (2 full MPG).

Cruising at 60 (5 MPH less than the typical Speed Limit) gets my best MPG while not causing as much of a hazard. If the road limit is 70 MPH, I will suck up the extra MPG loss and cruise at 65.

I have found there is a "floor" and in no wind conditions have never gotten much better than 13.5 MPG (at 55 MPH) towing our 5th wheel regardless.

in 4 years of towing this 5th wheel (no wind MPG) with our diesel Duramax:
55 MPH - 13.5 MPG
60 MPH - 12 MPG
65 MPH - 11 MPG
70 MPH - 9-10 MPG

In Kansas westbound a few years ago, we got 9 MPG at 60 MPH due to a very strong headwind. Horrible gas mileage.
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Old 07-10-2013, 09:21 AM   #28
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I also have to add that when calculating the effect of drag, the effect increases with the square of the velocity. A 10 MPH increase will have 100 times the increase in drag.
Herk, I will have to be Mr. Spock now. You are correct, the wind drag does follow a square law. But for a 100 fold increase in drag from a 10 MPH increase would be true if you were going 1 MPH and increased to 10 MPH. 10 ^ 2 = 100. An increase, say from 30 to 60 MPH, would be double, so by square law, 2 ^ 2 = 4, or 4 times the drag. Going from 40 to 60 would be 1.5x the speed, but would be 1.5 ^ 2 = 2.25x the drag.

I might add too, in your example, westward on I-70 through Kansas, not only are you sometimes subject to nasty headwinds, you gain about a half-mile of elevation between Salina and the Colorado line. On a steady climb, weight becomes a more significant factor on MPG. Of course, on the return trip, you should get some of that mileage back with a tailwind and slightly downhill.

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Old 07-10-2013, 09:33 AM   #29
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I know this topic is based on fuel mileage but with folks spitting out all sorts of speeds I just wanted to make sure that we are still watching the RPM gauge as well the Exhaust Gas Temp on the diesels. The average newer automatic transmission doesn't build a lot of line pressure under 1600 RPM meaning not much oil in the trans is flowing meaning that you may be burning off what is providing lubrication to the torque converter while at the lower RPMs... I do my best to never cruise at or below 1600 rpm. My goal is to be at 1900 RPMs while towing. The diesels are peaking for power anywhere from 1900-2100 RPM and that is going to be your most efficient place to be. Also with the diesel, for the most part, the higher the RPM the lower the EGTs will be, the cooler the motor will be and the happier the driver/owner will feel!
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Old 07-10-2013, 09:36 AM   #30
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Herk, I will have to be Mr. Spock now. You are correct, the wind drag does follow a square law. But for a 100 fold increase in drag from a 10 MPH increase would be true if you were going 1 MPH and increased to 10 MPH. 10 ^ 2 = 100. An increase, say from 30 to 60 MPH, would be double, so by square law, 2 ^ 2 = 4, or 4 times the drag. Going from 40 to 60 would be 1.5x the speed, but would be 1.5 ^ 2 = 2.25x the drag.

I might add too, in your example, westward on I-70 through Kansas, not only are you sometimes subject to nasty headwinds, you gain about a half-mile of elevation between Salina and the Colorado line. On a steady climb, weight becomes a more significant factor on MPG. Of course, on the return trip, you should get some of that mileage back with a tailwind and slightly downhill.

Bob
You are right about the drag, of course. However as any pilot will tell you, no matter whether you are traveling east or west, you will always have a headwind. Act of God and all...
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