Friday, November 07, 2008

Never Let Your Gas Tank Get Below Half Full

 
Friday's Urban Legend: True

How many of you have heard the story that you should always drive your car with the gas tank as full as possible? According to many, you should never let the gas in your tank fall below the half full mark on your gas gauge.

You probably thought this was a tale told by elderly wives—with apologies to old men who also tell tales.

Well, it turns out that there is actual, scientific, evidence to support this warning. Cliff Allen did the experiment according to the Sept. 13, 2008 issue of New Scientist [Petrol Gauge Challenge].
SLIGHTLY more practical routes to fuel economy occurred to Cliff Allen when he noticed that, according to his petrol gauge, the fuel in the top half of his tank lasted considerably longer than the bottom half. As any Feedback reader (and possibly only a Feedback reader) would, he investigated. Systematically.

Over several months he recorded the distances travelled using the fuel from the top and bottom halves. The average for the top was 400 kilometres (250 miles) and for the bottom a mere 300 kilometres (185 miles). Since then, he writes, "of course I have only used the top of my tank and have consistently achieved around 250 miles - I'm not stupid!"

Cliff was obviously keen to discuss this, at length, with his learned friends, "some of whom gained General Certificate of Secondary Education qualifications" at age 14. He was "mostly appalled at their incredulity and lack of interest".

However, his friend Alan suggested that the fact that petrol always comes out of the bottom of the tank causes it to use more petrol so we might benefit from turning the tank upside-down. John suggested the increased efficiency might be due to the height of the fuel, so the tank should be put on the roof. Mostyn proposed putting a brick in the tank, as this apparently works very well for saving water in toilet cisterns. Tony wants to make the top of the tank larger than the bottom, to increase the proportion of its volume at the top, and thinks a carrot shape would be optimal.


P.S. for the irony impaired.

12 comments:

  1. Irony noted, but:

    1) I've noticed that the "top half" of the tank appears to last longer than the bottom. I assume this is because the fuel gauge reading is not linear w.r.t. the actual volume in the tank, due to tank geometry and/or configuration of the linkage that translates float level into the electrical signal that drives the gauge.

    2) My father always said to keep the tank topped up *in winter*. The theory is that in the cold, condensation can form in the empty volume, leading to water in the gas and possible icing in the lines. However, most brands now seem to add alcohol to their gas in winter, which acts as an antifreeze (ie. by making any water miscible with the gasoline). They used to sell "gas-line antifreeze" (isopropanol IIRC) for this purpose.

    Since I'm talking to a biochemist, you can tell me if that last bit is plausible ;-).

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  2. 1) I've noticed that the "top half" of the tank appears to last longer than the bottom. I assume this is because the fuel gauge reading is not linear w.r.t. the actual volume in the tank, due to tank geometry and/or configuration of the linkage that translates float level into the electrical signal that drives the gauge.


    It was Allen's 5-year-old son who pointed out that the effect might be due to (we paraphrase) non-linearity of the petrol gauge. Where does he get such notions?

    I'd also like to note that, from my experiences, the guages are configured such that "empty" actually means "you can drive another 50-100 km".

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  3. On one of my cars, I can get 70 miles in before the gauge gets down to full. If I can just figure out how it goes that 1st 70 miles we may have the oil crisis licked.

    I had to post anonymously so that the government can't take this magical car from me.

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  4. On one of my cars, I can get 70 miles in before the gauge gets down to full. If I can just figure out how it goes that 1st 70 miles we may have the oil crisis licked.

    You think there might be a connection between this and what I described? Maybe we'll need to build a turnip shaped fuel tank...

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  5. I had heard a plausible reason to not let the fuel tank get TOO close to bone-dry-empty: particulate matter in the gasoline.

    Basically, the idea is that gasonline contains a tiny amount of grotty, particulate matter that can cause some mechanical troubles if too much of it gets into the combustion chambers at the heart of an engine. Running a fuel tank dry ensures that all of these particles get sucked into the gas line and into the engine, where they can cause some trouble. Keeping the gas tank at least half full means these particles are suspended in a large volume of liquid, and never reach high concentrations that can cause engine damage.

    The obvious answer to this rather silly urban legend is: "gas gauges are inaccurate".

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  6. I've long noticed that my mileage is much better when the tank is more than half full. But the other day, I drove a company car that does even better--at the end of the day, the needle was near Empty, but when I started the next day, it was back up to about 1/8.

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  7. I wonder how many of those friends got three GEDs?

    That would be impressive.

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  8. My car has one of those electronic computer thingies... you can set it to show fuel consumption, average speed or range (miles of travel remaining before the tank is empty) etc.

    If I have been driving around town in traffic and have half a tank of fuel left, the range is around 240 miles. If I then start a long motorway drive at a steady speed the range gradually increases to around 280 miles after 30 miles of driving. If only the tank was actually filling up...

    As to the original legend I suspect that with even a linear gauge the top half will last longer because most people will fill the tank until the pump cuts off (in the UK pumps stop automatically) and this is when fuel is in the filler tube - beyond the nominal 'full' point that the gauge is calibrated for.

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  9. woww, that really amazing

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  10. less stress on your fuel pump i would presume aswell

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  11. The test is easy if you drive any distance to work. With the tank filled to the top drive to work and back to fill up again. Record the gallons used. Use the same pump and drive the same route to establish a conststant gallons used. Once you have the gallons, set the tank at 1/2, drive the same route, using the same pump put that amount of gallons in. keep doing this to see if the gas level goes up or down. With everything equal it should always go back to 1/2.

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  12. Well I have learned that is has to do with the rate of evaporation. If the tank is half or more empty that means that is there is more room in the tank for the gas to evaporate more rapidly. And unfortunately for us all the gas pump was not designed to such fumes...only liquid. So by aiming to keep the tank as full as possible you actually get to use more of your money's worth in GAS!! ��

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