Fuel Economy and Mileage

Does a car use more gas when the tank is below half full?


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2015-04-13 14:55:44
2015-04-13 14:55:44

You do use more gas after the half mark because of evaporation. It's one of the first things you learn when you work with check emissions. This is why they test your gas cap as it allows a minimum amount of evaporation, but it does still make its way out of the tank without going through the engine.

Here is a quote from an experiment performed and published in New Scientist about why this happens:

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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