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Gasoline Tips Save Money: true?

  1. Sep 20, 2013 #1
    So this one is going around Facebook. I wanted to run it past my trusted scientifically literate friends here at PF before heeding any of the "advice."


     
  2. jcsd
  3. Sep 20, 2013 #2
    Sounds reasonable.
    But what worries me the most is how accurate the machines are when tallying up the amount you owe. How much of the gas I pay for is getting left in the hose? Or getting returned back to the reservoir? It seems like sometimes the money counter is going but no gas is coming out yet. I don't trust those machines.
     
  4. Sep 20, 2013 #3
    Before I used to pay with a debit card, my favorite thing to do was to see if I could squeeze out that last fractionally priced bit of gas without going over what I was paying. i.e. I would get to 10 dollars, and then give it that last little micro-pump that didn't quite cost another cent.
     
  5. Sep 20, 2013 #4
    I always wondered how much gasoline we waste as we take the nozzle out of the tank. There is always some drip. Overtime it must be a huge amount. A no drip nozzle shouldn't be so hard to make.
     
  6. Sep 20, 2013 #5

    jedishrfu

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  7. Sep 20, 2013 #6

    D H

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    I read that as Snopes calling every single one of those so-called savings tips into question.

    The only rule I try to follow is the half-tank rule, and it has nothing to do with saving a few cents. It has to do with getting the blank out of Dodge, ASAP. A quarter of a tank will not suffice when natural disaster strikes and you need to get out of Dodge, ASAP. You can't buy gas because the gas stations will either be closed or out of gas. A quarter of a tank will get you halfway out of Dodge because the freeways will be jammed, resulting in the worst gas mileage ever.

    I live where hurricanes strike, so I try to keep my tanks half full during hurricane season. For others, well, there are floods, forest fires, and of course zombie attacks. Ya never know when the next zombie apocalypse will strike.
     
  8. Sep 20, 2013 #7

    Drakkith

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    I don't worry about saving 5ish cents per tank. I just fill up whenever I need gas.
     
  9. Sep 21, 2013 #8
    ah, good old snopes. See, I trust the people on this forum so much I check here first.

    Well, overall it's not a big deal for me. I bike-commute to school and my wife has a 6 mile commute to work. We spend $30 a month on gas. But I was wondering about the science, or lack thereof, in the article.
     
  10. Sep 21, 2013 #9
    I bought 25l of 95e for 1.279euro/l today and paid 32euro for it. So yes, I lost 2.5 cents over 25l..a major calamity.
    Additionally I filled a 10l canister for which I paid 12 80 and it was perhaps half a shotglass shy of 10l.

    Must be an American thing then
     
  11. Sep 21, 2013 #10

    russ_watters

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    If the air takes all night to cool, how much do you really think a 1000 gal underground tank takes? Please provide your answer in months...
     
  12. Sep 24, 2013 #11

    collinsmark

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    Working
    working.png

    [Source: http://xkcd.com/951/]

    With mouse-over: "And if you drive a typical car more than a mile out of your way for each penny you save on the per-gallon price, it doesn't matter how worthless your time is to you--the gas to get you there and back costs more than you save."
     
  13. Sep 24, 2013 #12

    D H

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    That xkcd comic misses one thing: What if I blow out my car's fuel pump?

    Hurricane season is well past its peak, almost nothing to speak of so far. In a month or so I'll relax my "keep the gas tank at least half full" rule to my off-season "keep the gas tank at least a quarter full".

    From the perspective of a point of view that looks only at the time cost of money of filling a gas tank, it's best to drive the car until it is just short of empty and then fill it to the brim. While this minimizes the number of trips to the gas station, it increases the probability one will need to call a tow truck and have the car hauled to ones favorite mechanic. That last quarter tank is very hard on a car. That's where all the sludge settles, and even if the tank is sludge-free, pumping that last quarter tank out of the tank is hard on the fuel pump.
     
  14. Sep 24, 2013 #13

    AlephZero

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    Yeah, but the tips came from California. With all the high tech stuff they do in Silicon Valley, it stands to reason everything happens quicker over there :biggrin:

    At least in the UK, the price differentials between different gas stations are pretty stable over time, so the simplest cost saving is just avoid the ripoffs. The cheapest pump prices are usually the supermarkets, with further discounts off the pump price for their store credit cards etc, and most people visit them regularly anyway so there's no extra mileage involved.

    The places to absolutely avoid in the UK are motorway service areas, unless you are spending somebody else's money with a company credit card!
     
  15. Sep 24, 2013 #14

    Drakkith

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    Both of these things sound like a myth.
    For one, I don't think sludge builds up in your tank in this way.
    Second, how is the last quarter tank any harder on the pump than the first three-quarters?
     
  16. Sep 24, 2013 #15

    Office_Shredder

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  17. Sep 24, 2013 #16
    I've been an automotive mechanic for 10 years, and the tiny amount of sludge and water at the bottom if the tank is not worth worrying about, however, the gas is used as a cooling medium for the fuel pump, and going below 1/8-1/4 tank can cause the pump to overheat. Doing this repeatedly will cause the pump to fail. Trust me, I've made quite a few pay checks replacing pumps after someone ran low/out of gas.
     
  18. Sep 24, 2013 #17

    Drakkith

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    I'm assuming that everything's okay until you actually run out of gas? Even if it's just low, you still have fuel doing its thing in the pump?
     
  19. Sep 25, 2013 #18
    To an extent...

    The fuel pump is usually about 3/4 the height of the tank, so at 1/4 tank about 2/3 of the pump is exposed to air, with only the lower most portion being properly cooled (i.e. Keeping the temperature below 150 degrees F).

    When the gas level is low, the electric part of the pump warms up quite a bit and can burn out the coils. Actually running out does damage to the mechanical parts of the pump because the gas is needed to lubricate the gears, impeller, etc. and if you run out then you no longer have that lubrication, thus the internal components start grinding themselves apart.

    ImageUploadedByPhysics Forums1380160978.016129.jpg
    As this image shows, the electric motor in the fuel pump (which is the most vulnerable to overheating) is also the highest part of the pump, so it looses cooling first in a low fuel situation.
     
  20. Sep 25, 2013 #19

    russ_watters

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    Which I avoid completely by draining the tank every time, thus preventing either from building-up. :biggrin:
    Low is not out. This sounds like another myth. Unless the gas is circulated through the tank to cool the pump, the tank level can't affect the cooling capacity of the gas.
    Wait, what? Are you saying the fuel pump is inside the fuel tank?
     
  21. Sep 25, 2013 #20

    OmCheeto

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    Mine was. And that's why I pulled my external fuel pump, from car minus 4, hooked it up to my boats 6 gallon portable gas tank, and drove car minus one, to the Red-Green automotive shop about 12 blocks away.

    They were so impressed, they replaced my in-tank fuel pump, for cost.

    I really need to find those two guys, and thank them.
     
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