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Old 01-23-2009, 07:54 AM   #1
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Draft didn't work or fluke?

On a recent trip from Kansas to Minnesota I stopped in Des Moines for fuel. Des Moines is exactly halfway between my start and end. For the first half I tried to draft whenever possible, which was about 2/3 of the time. I was behind a pickup, a semi, and a minivan, and tried to stay about 3 stripes behind them. Each of these were generally driving between 68 and 70 mph and I was able to use the cruise for all of them except the semi. When I got fuel in Des Moines, I only got 17.5 mpg. For the second half, I just set the cruise on 70 without trying to draft at all, and achieved 20.4 mpg. I didn't make any more or less stops in either half, and the land is fairly stable over the entire trip (just small hills).

Why did I get such poor mileage drafting? Although perhaps I should say I got good mileage on the second half, as I normally average between 17 and 19.5 mpg when I have driven 70 historically. This is the first time I tried drafting. So the drafting was definitely the lowest number, which one would seemingly never expect.

BTW- on the return trip I filled at the same station in Des Moines, and got 17.6 mpg from Minneapolis to Des Moines and 18.6 mpg from Des Moines to Kansas. For the return trip I also just set the cruise at 70 without trying to draft at all. So the drafting was definitely the lowest number, which one would seemingly never expect.
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Old 01-23-2009, 08:03 AM   #2
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3 stripes will only work behind the semi. You'd have to get dangerously close to catch a good draft off a pickup or minivan.

What are the elevations of your start, end, and gas stop?
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Old 01-23-2009, 08:25 AM   #3
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Quote:
Originally Posted by theholycow View Post
What are the elevations of your start, end, and gas stop?
I was gonna ask that same thing.
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Old 01-23-2009, 02:09 PM   #4
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Yeah I did look that up earlier thinking that may be a culprit, but they are pretty close in elevation. KC is 740 feet, Des Moines is 820, and Minneapolis is 830 or so.
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Old 01-23-2009, 02:31 PM   #5
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Well, we can scratch that idea. Do you know what a 'normal' fill in your vehicle is from a certain point on the fuel gauge? In the Durango there is almost exactly 8 gallons left in the tank when the fuel light comes on so a normal fill should only be 17 gallons from that mark assuming I stop within a couple miles of that happening.

I'm wondering if you didn't get a higher than normal fill at your first half-way stop. That would explain a drop in MPG numbers and then a gain above normal for the second half of the trip. There is usually enough room for at least a couple more gallons in the tank and you could have easily been sitting at an abnormal angle for that to fill up.

Also, while unlikely, it does happen, but you could have also hit up a station that wasn't being very honest with their fuel metering. That got pretty popular when gasoline was $3-4 a gallon.
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Old 01-23-2009, 07:30 PM   #6
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miles in each direction??? this could be the difference in overfilling vs underfilling a tank. the angle of the car while filling is significant. as well as the quality of the fuel.
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Old 01-23-2009, 08:34 PM   #7
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I will speculate:

A pickup truck has a fairly large frontal area. The 3 stripes was in my Civic VX following a tractor trailer with the tandem (two trailer) hitch reinforcement between the rear wheels on the trailer, which blocks most airflow under the trailer itself.

You may have actually been in the area behind the vehicles where the air flow separation ends and this would actually create a higher pressure area that would cause you to have increased drag. Following closer may have helped, but it also is dangerous, reducing your margin of error if any debris or other objects (disintegrating tire) could really cause you some problems.

The host wind breaking vehicle really needs to have a significantly larger frontal area, as well as some part of the rear structure of the trailer that reduces airflow under the trailer to provide you with a significant improvement in mileage.

regards
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Old 01-23-2009, 09:18 PM   #8
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All good ideas here, thanks. It seems there are so many possibilities that we may never know for sure. I did fill up at the same station in Des Moines on the way there and on the way back, so the fuel quality should be the same. But I did use a different pump each time, and I have noted as much as a 2 mpg difference when I've used different pumps at my normal gas station (under similar driving conditions), so that is a definite possibility. My Explorer has a 20.5 or 21 gallon tank, and the gauge seems to run a little fast until the end, so it's pretty close to 5 gallons per 1/4 tank tick mark. But unfortunately I didn't think to look before I filled up and remember how much it should be filling. So I ruined that aspect.

Plus the point Gary brought up about being in the wrong position for drafting makes sense, and I actually thought about it while I was doing it. I figured I'd be better off moving closer to the host vehicle but I wouldn't feel as safe so I stayed back.

I know it sounds silly to you guys with smaller cars but I was so excited to have a 20 mpg tank in the winter. The air temperature on the way up started at about 50 in Kansas and ended at about 5 in Minneapolis, so having 20 mpg on the last half that probably averaged 10 degrees is awesome (pump aided or not!).
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Old 01-23-2009, 10:43 PM   #9
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yea the fuel gauge is not linear so you can never use that to estimate your mpg too well. such a difference makes me wonder if you were not drafting close enough and the effort you put into drafting actually made your mpg worse.

to be honest with you, if i got the results you did i would not try to draft again ever... but thats just me
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Old 01-24-2009, 04:19 AM   #10
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Quote:
Originally Posted by R.I.D.E. View Post
The host wind breaking vehicle really needs to have a significantly larger frontal area, as well as some part of the rear structure of the trailer that reduces airflow under the trailer to provide you with a significant improvement in mileage.
I thought that length is important too -- something like the draft extends up to 3 times the length of the vehicle. Of course frontal area is not to be discounted either.

Quote:
Originally Posted by KU40 View Post
I know it sounds silly to you guys with smaller cars but I was so excited to have a 20 mpg tank in the winter.
2 or 3 mpg saves a lot more fuel and money for you than it does for someone whose baseline is 30mpg. That's one of the shortcomings of measuring distance per fuel (MPG) rather than fuel per distance (l/100km, g/1000m, etc).
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