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Old 05-10-2012, 12:08 PM   #1
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Check my math

I recently bought a 2005 Lexington 283 by Forest River on a Ford E450 w/v10.

On a recent 1,000 mile trip I drove 64MPH one way and 59MPH back. Up I got 9MPG and back I got 10 MPG. I think this means that the slower speed took 30 minutes longer and saved me $22.40 at $4.00 per Gal.

What do you think?
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Old 05-10-2012, 12:24 PM   #2
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I didn't check your math, but I can save 1 mpg lowering my speed like you did. More Speed = More Wind Resistance = More Fuel. Keeping out of passing gear on those grades (no cruise) helps a lot too.
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Old 05-10-2012, 12:27 PM   #3
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Quote:
Originally Posted by artbode View Post
I recently bought a 2005 Lexington 283 by Forest River on a Ford E450 w/v10.

On a recent 1,000 mile trip I drove 64MPH one way and 59MPH back. Up I got 9MPG and back I got 10 MPG. I think this means that the slower speed took 30 minutes longer and saved me $22.40 at $4.00 per Gal.

What do you think?
At 10 MPG you used 100 gallons of fuel, at 9 MPG you used 111.11 gallons of fuel. the difference of 11.11 gallons times $4.00 a gallon equals a savings of $44.44. Enough for a case of beer or two and without a doubt.

The trip at 64 MPH would take you 15.62 hours and the trip at 59 MPH would take you 16.94 hours, a difference of 1.32 hours or about 1 hour 20 minutes longer for the 59 MPH trip. Although I can't speak for you, since I am retired and have all the time I want to make the trip I would go with the slower speed and buy the two cases of beer to drink for after I am set up.
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Old 05-10-2012, 12:35 PM   #4
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On perfectly flat ground, with no traffic issues, and calm winds, it looks right, but there are more variables at play.

I'd ask what the elevation of the beginning and end points were, what were the winds doing? What was traffic like? Were you drafting a big rig part of the way? Did you take the same route?
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Old 05-10-2012, 12:53 PM   #5
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Thanks for the comments. On the trip up I had to climb up and over a 2000' ridge which I avoided on the way back. As to drafting, I watched the Mythbusters show where they could get no advantage until they were within about five feet of the semi truck in front of them. As to winds, They may have contributed to the better mileage.

Here is an interesting web page showing wind across the USA, you can zoom it too:
Wind Map
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Old 05-10-2012, 01:08 PM   #6
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Thanks for the comments. On the trip up I had to climb up and over a 2000' ridge which I avoided on the way back. As to drafting, I watched the Mythbusters show where they could get no advantage until they were within about five feet of the semi truck in front of them. As to winds, They may have contributed to the better mileage.

Here is an interesting web page showing wind across the USA, you can zoom it too:
Wind Map
Wow! Thanks for that link!

Mythbusters Big Rig Drafting Data indicated a 11% increase at 100 feet, a 20% MPG increase at 50 feet, and a 39% increase at 10 feet. All too close to follow in an RV, but still significant.

Hard to say but I'd guess the avoided ridge and the tailwinds probably contributed as much to your fuel economy than your 5MPH speed decrease.
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Old 05-10-2012, 11:32 PM   #7
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Wind and gradient have more impact on fuel consumption than speed. IMO, if a $22 difference on a 1000 mile trip is a major concern, maybe RVing isn't for you.

If at first you don't succeed, maybe you shouldn't try sky-diving.
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Old 05-10-2012, 11:49 PM   #8
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Quote:
Originally Posted by Ramblin View Post
On perfectly flat ground, with no traffic issues, and calm winds, it looks right, but there are more variables at play.

I'd ask what the elevation of the beginning and end points were, what were the winds doing? What was traffic like? Were you drafting a big rig part of the way? Did you take the same route?
My thoughts exactly as I read the OP. I remember getting incredible MPG on about a 800 mile leg of a trip last year and then realizing that we had lost a great deal of elevation over that stretch without it being very noticeable. I can certainly see improved MPG with lower speeds as well but the elevation is a big deal too.

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Old 05-11-2012, 02:00 PM   #9
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While it's tempting, you really can't make a decision based on some exact'ish science and some widely variable factors.

For instance, the old line "I had to walk 5 miles to school, uphill both ways" comes to mind, if you drove 500 miles upwind, uphill, or both all those careful calculations go out the window.

In my own experience, I can get 1+ mpg difference on exactly the same route, and at the same speed, just by having the wind change directions, in my face, or at my back.

If the speedometer reads say 60 mph exactly and there's a 25 mph wind, and you're going exactly upwind and then downwind, the motorhome will have an "airspeed" with a 50 mph spread.

That has a bunch more effect than most give it credit for.

Temperature and humidity also have a noticeable effect on fuel mileage
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