Water Weight and Miles per Gallon - Fiberglass RV


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Old 07-10-2014, 02:45 PM   #1
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Water Weight and Miles per Gallon

I came across this recently.... suggesting that carrying water has little effect on miles per gallon.

"Hereís a really simple example. How many times have you heard that reducing weight in your trailer will improve your towing fuel economy? Like a lot of things, thereís some truth to that, but not nearly what people think. Sure, lowering the trailer weight will result in less energy needed to get the trailer moving from a stop, or pull it up a hill. But RVíers spend most of their fuel budget pushing air out of the way, not pulling away from STOP signs. Aerodynamics play a much larger role in fuel economy than weight.

The misconception about the impact of weight has led to the popular myth that you can save fuel by not carrying water in your fresh water tank. Itís nonsense, but it is continually spread even by experienced trailerites."
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Old 07-10-2014, 03:07 PM   #2
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Originally Posted by honda03842 View Post
I came across this recently.... suggesting that carrying water has little effect on miles per gallon.

"Hereís a really simple example. How many times have you heard that reducing weight in your trailer will improve your towing fuel economy? Like a lot of things, thereís some truth to that, but not nearly what people think. Sure, lowering the trailer weight will result in less energy needed to get the trailer moving from a stop, or pull it up a hill. But RVíers spend most of their fuel budget pushing air out of the way, not pulling away from STOP signs. Aerodynamics play a much larger role in fuel economy than weight.

The misconception about the impact of weight has led to the popular myth that you can save fuel by not carrying water in your fresh water tank. Itís nonsense, but it is continually spread even by experienced trailerites."


Norm,
This sounds like good advice to me! I can see where aerodynamics would play a large part in the use of fuel. So moving air burns more fuel! One can see on most 18 wheels they have shields that deflect air and they save fuel!
So, are Scamps the most aerodynamic travel trailer?
Just wondering?
Thanks
Carl
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Old 07-10-2014, 03:27 PM   #3
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I totally agree that its the drag that kills us. I get better gas mileage towing my 17' Casita with its curved front than I did towing my 13' Ventura with its flat surfaces. The Casita weighs almost twice as much. I can't even imagine towing one of those monster 5th wheels around.

The previous owner of the Ventura warned me about the mileage but I thought it can't be that bad "its only a 13' trailer".

I had thought about an air deflector but the on line reports said they did not help. I can see why when you are trying to pull a 8 x 10 square brick through the air.
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Old 07-10-2014, 03:36 PM   #4
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Norm is spot on. Wind resistance increases exponentionally rather than proportionately with increases in speed. It takes much more power to overcome wind resistance going from 60 70 mph than it does going from 50 to 60 mph.

I like my Casita, because it is rounded in front and has non roof mounted A/C. That having been said, I am trying to think of ways to improve the MPG. So far, just driving no faster the 60 has been the best fuel saver.


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Old 07-10-2014, 05:53 PM   #5
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When a passenger rides with you does it effect your miles per gallon? Probably not!

A person probably weighs more than any water you'd put in the trailer.
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Old 07-10-2014, 07:48 PM   #6
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I disagree. This may be true for long hauls on the flat, but I drove from Fresno to Seattle in early June and again early July, both times loaded, both times, getting about 17-18 mpg. I drove from Seattle to Fresno in late June, completely unloaded (I never carry water, but I took everything out.) (The car was also loaded going north, unloaded going south.) I got 20-21 mpg unloaded. Now, it may be downhill to California, but I started at sea level in the north and ended up at about 100 feet above sea level in the south, so it's pretty much the same elevation gains. It is, however, substantial up and down hill, which means every pound carried has to be lifted up those hills. And I really felt the difference on those hills!

I'm sure carrying only a little water won't make much difference, but it makes some. I probably carried/didn't carry around 400 lbs, the equivalent of 50 gallons of water.

I will also say, the trailer did not tow as well empty- it needed a little more tongue weight than it has with everything out of the front ( probably because without removing stove and fridge I couldn't take everything out of the back.)
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Old 07-10-2014, 08:31 PM   #7
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Water as a percentage of system weight.

"This may be true for long hauls on the flat",

Bobbie,
I should have extended the definition of water to fresh water. Most of us carry less than 20 gallons of fresh water. Few of us travel with full gray and black tanks or even anything in them. I would say few travel with full fresh water tanks. I Typically carry 6 gallons half of capacity.

Weight is important on hills, however 160 pounds of water (20 gallons) is relatively trivial considering that the typical weight of the tow vehicle and the trailer is about 7000 pounds. In this case 160 lbs of water only represents 2% of the combined tow vehicle and trailer. Even 50 gallons represents about 5% and most peoples typical water load represents 1%

There are three typical engine loads: air resistance, system friction and weight.

Weight mostly comes into play moving up hills and when accelerating. For most speeds the overwhelming load is air resistance.

From the above carrying any practical quantity of water makes a very small difference to the engines requirements, again even 50 gallons, though an unlikely amount only represents 5%.

As to tongue weight it certainly can be too low, causing the trailer to feel light. We tow at about 7-8% tongue weight without problem.

One thing that can cause a problem is weight at the rear of the trailer. In our trailer we attempt to keep significant weight over the axles and have not experienced sway or lightness.

Hope that is an improvement over my original statement.
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Old 07-10-2014, 08:37 PM   #8
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I read it is 1% for a 100 lbs but only if you have to stop and restart, it will take more energy to get going. For those who take non interstate routes with traffic lights, weight will reduce you efficiency more than wind since you probably are not going fast enough.
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Old 07-10-2014, 09:48 PM   #9
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Originally Posted by cpaharley2008 View Post
I read it is 1% for a 100 lbs but only if you have to stop and restart, it will take more energy to get going. For those who take non interstate routes with traffic lights, weight will reduce you efficiency more than wind since you probably are not going fast enough.
I agree with that generalization. I used to tow glider trailers long distances behind a minivan. In town definitely worse mileage, but on the interstates it actually improved mpg.

Those trailers are about 2000# and very long but with a frontal area less than the minivan.

YMMV

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Old 07-10-2014, 10:00 PM   #10
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I agree with that generalization. I used to tow glider trailers long distances behind a minivan. In town definitely worse mileage, but on the interstates it actually improved mpg.

Those trailers are about 2000# and very long but with a frontal area less than the minivan.

YMMV

Jim
That explains why I got better mileage when I took the ride to ParkLiner last fall, all on the interstate (except goin around Baltimore)...then when I take a 64 mile ride to my friends house where its lots of stop and go traffic lights. Interesting.
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Old 07-10-2014, 10:23 PM   #11
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Norm,
This sounds like good advice to me! I can see where aerodynamics would play a large part in the use of fuel. So moving air burns more fuel! One can see on most 18 wheels they have shields that deflect air and they save fuel!
So, are Scamps the most aerodynamic travel trailer?
Just wondering?
Thanks
Carl
Certainly a popup or a small teardrop would push less air than a Scamp, because of their smaller frontal area. Other than something along those lines, Scamp and Casita and Boler are right up there. I would speculate that perhaps the Bonair Oxygen could do a bit better aerodynamically; it was tapered like a fish's body.
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Old 07-10-2014, 10:45 PM   #12
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When I built my teardrop sized lil vardo I averaged about 12mpg towing it... it was 6'6" wide (or so)and had 54" interior headroom and 10 feet long. I moved up to my ParkLiner and get around 14.5 to 15 mpg. The wind resistance towing the lil vardo was horrible. I gave thoughts to some kind of rooftop wind deflector.
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Old 07-10-2014, 10:53 PM   #13
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Sheesh..... Starting and stopping in town is always hard on gas mileage. Look at the estimated mileage listed on vehicles. My truck was 15 mpg in town and 20+ on the highway. That's without trailer and ethanol.
Water weight is less than an additional passenger as Norm said. My Scamp has a 12 gallon tank that is full when I leave if possible. (Some places don't have water or dump station). That 12 gallons is about 96 lbs. Not enough to fuss about.

Sometimes have to think about the backpackers that drill out the handle on their tooth brush to save a fraction of oz in weight.
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Old 07-11-2014, 07:20 AM   #14
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Some good posts here. The thing about water too is that it's carried low in the trailers. With the weight there it keeps the center of gravity low which can benefit towability.

I have read about of a number of folks who tow with full water tanks because it improves the way the trailer handles and tows.
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