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.

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P.S. for the irony impaired.