ron.dittmer
Northern Illinois
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I read years ago that filling a gas tank at day break compared to mid or late day, you get an automatic 10% improvement in fuel economy. It has to do with the natural changes in barametric pressure and how that affects the contraction/expansion of the fuel itself.
I don't know if it is fact or fiction. Maybe 10% is too generous a figure. Does anyone here know more about this? If true, this should be made common knowledge.
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gbopp
The Keystone State
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I saw an article that said it's a myth. The underground storage tanks are at a constant temperature, so is the fuel. So it doesn't make a difference what time of day you fill up.
Another article said to buy in the morning not because of the temps but, because most stations raise their price in the afternoon.
Just telling you what I read, don't shoot the messenger. 
Snopes article.
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garym114
Bluff Dale, Texas
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This is on the misbelief that the fuel is cold at night, more dense, and warmer during the day, less dense. This is an urban myth.
Fuel is stored in underground tanks where the temperature is constant and the density does not change.
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LARRYAG
Delaware
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I think the theory is based on the differences in temperature of the fuel. Gas is stored in underground tanks that maintain a constant temperature of lets say 60 degrees. If you get gas on a hot summer afternoon (say in the 80's or more), the 60 degree gas will expand and in theory give you more gas. However, as you can imagine, there are a lot of variables in that theory, such as even though the gas you bought has expanded (in theory), are you driving when that gas is in the expanded form, i.e. are you driving early the next morning when the temperature of the gas in your tank has fallen to morning temperatures and thereby contracted back to near the temperature (and thereby volume) you bought it at. Also, even though the gas may have expanded... does expanded gas produce the same energy as the contracted gas. If not, then you are rally no better off. To illustrate the point, if the gas expanded to twice the volume, but only produced half the energy, did you really gain anything. IMHO... just pay up and direct your energies toward voting in those folks that endorse the production and use of our own domestic resources. I for one am disgusted at sending so much of our dollars to people who hate us. Sorry, I am a diesel owner and still can't understand why diesel fuel is priced higher than regular fuel, when it costs less to manufacture diesel than gasoline. This whole fuel situation just doesn't pass the smell test. Sorry, just my 2-cents.
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sch911
Rochester Hills, MI
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Joined: 04/13/2003
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If you want to save fuel, slow down. This tactic will work 100% of the time.
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ArcticDodge
Sammamish, WA
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I highly doubt the added fuel eco theory but I have had fuel weeping out of the filler on a warm day after fillling up in the morning. So yes, the gas can and does expand when the temperature increases.
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Harvard
51.4N 114.4W
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Joined: 12/24/2005
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ArcticDodge wrote:I highly doubt the added fuel eco theory but I have had fuel weeping out of the filler on a warm day after fillling up in the morning. So yes, the gas can and does expand when the temperature increases.
Yes this is true BUT the BTU content is based on the MASS (weight) and the mass does not change if there is no loss of fuel due to leakage.
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Frosty's Mom
Treasure Valley, SW Idaho
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sch911 wrote:If you want to save fuel, slow down. This tactic will work 100% of the time. oh i agree-but ya gotta get the DH (driver) to hear yah!
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hotbyte
Barnesville GA
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I've always heard 2 sure fire methods to increase mileage...1) Put an egg under your gas pedal and don't break it while driving and 2) Always go downhill 
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pnichols
Santa Cruz Mountains
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Quote:If you want to save fuel, slow down. This tactic will work 100% of the time.
The trouble is .... you can't slow down enough to get 12 MPG.
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