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Old 09-11-2008, 02:20 AM   #1 (permalink)
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is it true that a full tank gives better mileage?

I received this article via email. I don't know if you would agree. any feedback guys? the debate here is extra weight vs the evaporation of gas!
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Pumping gas.... good things to know.

I've been in petroleum pipeline business for about 31 years, currently working for the Kinder-Morgan Pipeline here in San Jose , CA . We deliver about 4 million gallons in a 24-hour period from the pipe line; one day it's diesel, the next day it's jet fuel and gasoline. We have 34 storage tanks here with a total capacity of 16,800,000 gallons. Here are some tricks to help you get your money's worth:

1. Fill up your car or truck in the morning when the temperature is still cool. Remember that all service stations have their storage tanks buried below ground; and the colder the ground, the denser the gasoline. When it gets warmer gasoline expands, so if you're filling up in the afternoon or in the evening, what should be a gallon is not exactly a gallon. In the petroleum business, the specific gravity and temperature of the fuel (gasoline, diesel, jet fuel, ethanol and other petroleum products) are significant. Every truckload that we load is temperature- compensated so that the indicated gallon gage is actually the amount pumped. A one-degree rise in temperature is a big deal for businesses, but service stations don't have temperature compensation at their pumps.

2. If a tanker truck is filling the station's tank at the time you want to buy gas, do not fill up; most likely dirt and sludge in the tank is being stirred up when gas is being delivered, and you might be transferring that dirt from the bottom of their tank into your car's tank.

3. Fill up when your gas tank is half-full (or half-empty), because the more gas you have in your tank the less air there is and gasoline evaporates rapidly, especially when it's warm. (Gasoline storage tanks have an internal floating 'roof' membrane to act as a barrier between the gas and the atmosphere, thereby minimizing evaporation. )

4. If you look at the trigger you'll see that it has three delivery settings: slow, medium and high. When you're filling up do not squeeze the trigger of the nozzle to the high setting. You should be pumping at the slow setting, thereby minimizing vapors created while you are pumping. Hoses at the pump are corrugated; the corrugations act as a return path for vapor recovery from gas that already has been metered. If you are pumping at the high setting, the agitated gasoline contains more vapor, which is being sucked back into the underground tank, so you're getting less gas for your money. Hope this will help ease your 'pain at the pump'

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Old 09-11-2008, 03:31 AM   #2 (permalink)
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Truth!!!

I'm not gonna read your entire post - I'm just going by the title, okay?

If you look in my fuel log, you'll see that I tried running partial fills for a while - albeit on a near empty tank - and my FE dropped like a rock!

I haven't tried partial fills on a near full tank yet - but I can tell you that running my tank low gave me mileage for sh!t, if that helps...

In other word(s) - FAIL!!!

Now, I'll read your post...
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Old 09-11-2008, 03:38 AM   #3 (permalink)
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Okay, I read your copy n' paste...

Here's my judgement, and you can take it from the smartest guy on this web site!
  1. False
  2. False
  3. True
  4. True
And, if you don't think I'm smart, look at the BBCode I used for this message...
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Old 09-11-2008, 05:20 AM   #4 (permalink)
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Ok, ego aside....

I have noticed a very different result with my Metro. I get better FE the further down I ride the tank before filling up again. I do not know why BDC and I would have such different results. It could be a weight factor for my Metro, or a different fuel delivery system.

DISCLAIMER: I do NOT use scangauge, so my results are pump based...
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Old 09-11-2008, 05:53 AM   #5 (permalink)
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Quote:
Originally Posted by Will View Post
I do not know why BDC and I would have such different results...
Um...

See bullet point #3 above, Will.

Maybe your gas doesn't evaporate like in AZ - one of the hottest places on Earth!
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Old 09-11-2008, 05:58 AM   #6 (permalink)
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No, Tennessee is not near as hot as AZ, but I would not think that would be the total deciding factor...
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Old 09-11-2008, 06:04 AM   #7 (permalink)
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Quote:
Originally Posted by Will View Post
No, Tennessee is not near as hot as AZ, but I would not think that would be the total deciding factor...
Well, I suppose I could recreate the experiment with a semi-full tank, instead of near empty - for 1000 miles (fill it up every day for two weeks), and I'll report back.

I tried it on an empty tank 'cause I was trying to save weight, but my mileage took a noticable down-turn!
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Old 09-11-2008, 06:21 AM   #8 (permalink)
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Maybe you're getting better mileage on shorter tanks or longer tanks because of how your average mileage is working. If you do all highway driving on the first half of tank, your FE will obviously be higher than if you do hwy the first half, then city the second. Same thing with Will. Averages?
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Old 09-11-2008, 06:53 AM   #9 (permalink)
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I only use my Metro in commute to and from work, so my % never changes. I am at 70% state road and 30% city.
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Old 09-11-2008, 07:25 AM   #10 (permalink)
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It seems counter-intuitive that you would get better gas mileage on a full tank. As the fuel in the tank is being burned, the car is decreasing in weight (unless the occupant is eating a bunch of Big Macs.)

So it stands to reason that the car would get better economy while the tank was getting lighter and lighter. A gallon of gas weighs approximately 6 pounds (2.72kg), my tank holds about 13 gallons, and I usually fill up 10+ gallons at a time. If my math serves me correctly, this translates to a 60+ pound (27.2kg) swing in weight. Last time I checked, reducing the weight in the car increased economy.

I don't have any experience with repairing gas tanks or fuel pumps, but unless the pressure on the fuel pump at a full tank is significantly higher or lower than at half or lower, or the percent weight distribution has a major affect mileage, then there would be absolutely no reason to believe that you get better economy on a full than a partial.

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