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After you are done putting gas in your car, is there any leftover gas in the hose between the nozzle and up to the level of the tank opening (below the red line in the image below)? To be clearer, think about the "communicating vessels" concept.

A follow up question, do gas pumps use pressure to ensure that all gas is pumped in the car tank and no gas is left in the hose?

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    I'm voting to close this question as off-topic because it does not appear to be about motor vehicle maintenance or repair. – Jason C Aug 4 '16 at 21:21
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    Oh come on, it's still a great question, where else does this belong? Don't be so stringent, each of us probably wondered whether pulling the hose up will yield us the fuel in the hose. Some of us still do. – I have no idea what I'm doing Aug 5 '16 at 9:29
  • @IhavenoideawhatI'mdoing "Some of us still do" - If you can't visualize it, it's easy enough to try next time you fill up! One experiment to try is wait for the pump's flow control valve to shut off (like when you've hit your pay limit) then hold the nozzle open and lift it. I do not know if the nozzle would allow back airflow into the hose with the pressure off. :) – Jason C Aug 5 '16 at 13:53
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It doesn't really matter if there's gas left in the hose. The only way for all gas to flow out of the hose is for the hose to collapse or air to enter it. It's metered at the pump side but it doesn't matter, because flow out of hose = flow into hose. Also remember that it's pressurized from the pump side (it's actually pressurized from a pump in the underground tank), it doesn't just get like, passively siphoned out of the tank in the ground and into your car.

Flow through the hose stops at the nozzle when you either let go of the handle or liquid covers its venturi inlet. There isn't (ideally) any air in the gas coming through the hose from the pump (actually I don't think there's even allowed to be air for it to pass weights & measures certification, but don't quote me on that: here's the measurement design system guide for the UK, at least which you are welcome to study).

This means that any gas that comes out the nozzle must create suction in the hose which pulls more gas into the hose and more through the meter (and probably won't pull any gas through at all if the dispenser's check / flow control valves are shut off), so the hose won't empty. Also, any gas that goes into the hose from the pump pushes gas out the end of the hose (should take about 2 ms for a pressure change at one end of an 8' hose to be reflected on the other, if you are curious).

So what comes out of the nozzle is precisely metered, and what's left in the hose comes out when the next person pumps gas, and is also metered. There is no reason at all to make sure the hose is empty.

Lifting up the hose when you're done doesn't really get much gas, and it only comes out of the nozzle not the hose. With the handle off (nozzle closed) the only extra gas you're getting out of it is those last few drops left in the end of the nozzle downstream of the nozzle's venturi gate (which is near the handle), because the nozzle is sealed off from the hose at this point. The 0.1L figure in the other answer seems reasonable if you picture the volume of the nozzle. That's about 0.026 gal = about 7.2 cents worth of gas (according to current average gas price in Hawaii, US's most expensive gas state). For a 12 gal fill-up you save about %0.22 of your money, but a penny saved is a penny earned, I guess. And any added value of lifting up the hose this way assumes that the gas left in the downward pointing nozzle didn't already drip out into your tank anyways.

  • ...except in the case where the previous user lifted the hose up to shake out the wee bit contained in the hose :-) – Rory Alsop Aug 5 '16 at 17:02
  • @Rory Or the previous customer's nozzle leftovers just evaporated. Sniping gas pumps for leftover nozzle drippings sort of reminds me of searching phone booths for forgotten change. Desperate times need desperate measures I guess, lol. – Jason C Aug 5 '16 at 17:15
  • hahahaha - yep :-) – Rory Alsop Aug 5 '16 at 17:16
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Yes, it mostly is filled up with fuel. That's why when I was a moped-warrior I always lifted the hose up after filling, to make sure I get extra fuel :) You only have to prepay or set it to a fuel amount you want to buy, so it stops pumping.

  • How much extra fuel do you get from this? :) – tlhIngan Jul 22 '16 at 19:21
  • Well, for a moped it was a lot :D Maybe 0.1L, maybe less. It was years ago... – Arturs Bolsunovskis Jul 22 '16 at 19:38
  • I always lift the loop of hanging hose and give it a bit of a shake too. You only get a tiny amount of fuel but at todays prices, even a tiny bit of fuel is better in your tank than left in the hose. – Steve Matthews Aug 5 '16 at 9:38
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of course there is a reason why someone would want to know how much gas is left in the hose someone buying premium would be getting regular or mid grade which is a lower octane. Or, someone who’s using gasoline for small engines and wants pure gasoline in the case of a pop that offers it would also be getting ethanol mixed into his gasoline for his small engine. The more gasoline remaining in the hose The more alcohol will be going into the camp.

  • I don't know about other countries, but here we have a separate hose for each grade of fuel - and the ones for Diesel are physically different to the petrol ones to reduce the chance of misfuelling – Nick C Aug 5 '18 at 15:58
  • Most stations in the United States use a common hose shared for 3-grades of gasoline and a separate hose for diesel fuel. – Waitman Aug 14 '18 at 0:16

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