# Why is gas(oline) in gas stations sold by volume (as opposed to mass)?

Fluids (including natural gasoline/petroleum) have variable volume based on the ambient temperature for the same mass of fluid. So, really, the amount of gas that you're filling your car with depends on the temperature because it's not the volume of fuel that makes your car run, but the mass which is combusted. In aircraft, aviation fuel is always measured by the kilogram. So, why is it measured by volume for cars?

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i often see at filling stations x price/kg and a pressure indicator at 200bar. – Waqar Ahmad Oct 9 '13 at 6:37
Link en.wikipedia.org/wiki/Gas_pump#The_metrology_of_gasoline Most pumps also indicate the fuel density. – user80551 Oct 9 '13 at 6:38
Because it's much easier and cheaper to measure volume than it is to measure weight? – John Rennie Oct 9 '13 at 6:39
Because a car's fuel tank has a fixed volume? – Pranav Hosangadi Oct 9 '13 at 7:32
Well, you can apply the same argument to a aircraft's fuel tanks, though weight is usually a much bigger concern than having fully filled tanks... – shortstheory Oct 9 '13 at 10:48

I take it that the question being asked by OP is :

"""...Why is gas(oline) in gas stations sold by volume (as opposed to mass)?..."""

The answer is simple. Gas stations get their gasoline delivered by "jobbers" who can supply them with gasoline that can come from a variety of different sources; even from different manufacturers. Gas stations are equipped with simple equipment that can measure fluid VOLUME, and as most liquids, are of low compressibility (not zero), that is a fairly reliable measure (for commerce).

No gas station is equipped with any kind of equipment that can measure MASS which is a rather complex physical parameter to measure, and requires highly specialized equipment.

On the other hand, If I have read the question incorrectly, then forget that I gave an answer.

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This looks like a reasonable explanation. But I'm sure it's not impossible to make a system designed for calculating mass dispensed by taking specific gravity and volume into account. – shortstheory Oct 10 '13 at 16:01
Mass; even under laboratory conditions, is commonly "measured" with a mass balance, which directly matches the weight of one body against the weight of some known mass body(ies). It is presumed that the gravity is the same under both balance pans, so it cancels out. As a result the method requires some digital summation set of standard masses to form any combination of mass, and that has to be done over the whole range of masses to be measured. And the mass balance needs to be able to withstand the weights of those masses. Any other method presumes knowledge of local (g) – user26165 Oct 11 '13 at 2:40
So you propose a method to dispense a known mass, by taking into account the (to be measured) volume of the specimen. Simply wonderful; why didn't I think of that. – user26165 Oct 11 '13 at 2:41
I guess it's safe to assume that the acceleration due to gravity is consistent in all places on Earth's surface. IMO, just simple weighing scales would suffice! – shortstheory Oct 11 '13 at 7:54
Well shortstheory, it most definitely is NOT safe at all to assume that; in fact it is quite untrue. In 2007 an international sailboat racing series ran into a problem, when the series moved to a sailing site in Sweden. Everyone of the boats failed measurement, when they all tested overweight; and all had to be modified to pass. The problem was the weight of the boats was specified in the rules; not the mass; and Sweden has higher gravity than tropical places. – user26165 Oct 11 '13 at 22:45

CNG fuel dispensers indeed measure mass not volume. Mass flow meters (coriolis)do that job. Pakistan has the biggest number of CNG vehicles as its a bit low cost(India is lagging behind Pakistan in CNG:) @Qmechanic wrongly edited your question.

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First of all in aviation (from my previous work in this field) fuel usually measured by "US Gallons" which is a measure of volume, anyway usually in big tenders sometimes they measuring it in Tons (for a what called Jet A-1 class of this fuel -used for aircrafts- it is 1 Ton=331.3 US Gallons if I remember it right).

And regarding natural gas, the fact that up to a very high precession, most liquids can't be compressed, thus if you compress the gas to convert it into liquid, and knowing the density, you can always calculate it's weight, and this is very comfortable because it's much more efficient to transport gases in their liquid form.

Also considering natural gas heat capacity and temperature expansion coefficient, in usual temperature conditions it's volume changes in a very small amount, and I believe that it's much easier to dial with liquid gas in the car's engine than in it's normal state.

Finally, in your place, I would worry more about the "Energy conversion efficiency" of the gas, because usually the natural gas comes with a lot of other gases, that are not useful -in sense they can't burn in usual conditions-, this may cause your car to walk 50 km instead of 200km using same amount of gas.

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... gases can't be compressed? – Emilio Pisanty Oct 9 '13 at 10:40
I also learned about aviation fuel being measured in kilos from this site Could be wrong though. – shortstheory Oct 9 '13 at 10:54
@EmilioPisanty: Please read more carefully, I was talking about liquids... – TMS Oct 9 '13 at 19:37
@shortstheory I dialed with big airlines and Cargo aircrafts, I think those guys talking about private aircrafts which some times uses other classes of fuel, anyway I think it's not relevant to your question. – TMS Oct 9 '13 at 19:40
@TMS so say liquids instead of fluids. – Emilio Pisanty Oct 9 '13 at 19:48