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Filling petrol in the day or night

  1. Apr 18, 2008 #1
    my friend told me the other day that it makes more sense to fill petrol in your car in the night instead of in the day.
    he says, that since its generally cooler in the night, the volume of petrol you get is more than what you would get if it were daytime instead, since its warmer. so u get slightly more fuel and a better mileage.

    my take is this : whether day or night, the amount of petrol(moles) you fill in your car doesnt change. so it doesnt make any difference.

    what makes more sense?
  2. jcsd
  3. Apr 18, 2008 #2


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    In theory, your friend has the right idea, but in practice, it doesn't work that way. Most gas tanks are buried in the ground, and the gasoline is insulated well enough that it won't change temperature noticeably over the course of the day or night. Fill up your car whenever you want - it won't make any difference.
  4. Apr 18, 2008 #3

    D H

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    Winter, at least in the USA. Energy content is a proportional to fuel mass, not fuel volume. US fuel stations charge per unit volume, not per unit mass. Since fuel density decreases as temperature increases, it is better to buy fuel when temperature is lower. YMMV (literally!) in other countries. Some countries mandate a temperature-based adjustment to the measurement of the quantity of delivered fuel.
  5. Apr 18, 2008 #4
    I'm not sure how the volume of the fuel is measured. If the device would always measure the correct volume, then it would make more sense to buy dense petrol. But maybe it is not so: maybe the volume flow is calculated from measured value of the speed of the petrol? Then higher temperature would also increase the cross-section of the tube, so you could get more volume for the same price.
  6. Apr 18, 2008 #5


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    Your friend is right. However, as Turbo has stated, the temperature of the fuel at the point being metered generally doesn't change much, so only a very small difference would be measurable (and probably not noticed).

    Lojzek's theory is nice but not correct. Fuel pumps don't calculate flow from fluid speed; they measure the flow directly (volumetrically) using a flow meter.
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