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I ride a vehicle with a small fuel capacity - my typical fill-up is about 3 quarts. What I want to know is if I'm filling up at a pump that has one hose for all grades do I get the grade I select, or is a significant part of my fill-up whatever was left in the hose from the last customer? Ultimately, this is a question about where the mixing happens, and how much gasoline is between the point of mixing and the end of the hose. Are gasoline pumps like those soda dispensers at bars that mix in the handle/nozzle? If not, how much gas containing hose is between the handle and mixer (inner diameter and length are fine - I can take it from there)?

For example, if the hose inner diameter is 1 centimeter then the area of the hose's opening is about .12 square inches, and if the distance from handle to mixer is 4 meters (about 13 feet), the hose volume is about 0.314 Liters, or about 1/3 of a quart. In this case, that wouldn't be a huge impact on a fill-up of 3 quarts. If the inner diameter is 2 centimeters, though, then the hose volume is about 1.3 quarts, a big, though not overwhelming, impact.

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  • That is a very interesting question, just wondering if your vehicle requires a high grade octane fuel and you are worried about the lower from the nozzle? – method Mar 16 '17 at 6:36
  • Manual recommends a higher grade octane, I'm a tad neurotic about it, and at a 30-ish cent difference on the tank, why not (if it might actually help)? – Sean Lake Mar 16 '17 at 6:40
  • I guess at least the amount of fuel in the nozzle will be from the previous fill up, because I doubt there are multiple hoses inside the main one. – oryades Mar 16 '17 at 6:45
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    Are you in the US? I've noticed this and thought about this problem when visiting US fuel stations. Here in the UK, each variety of fuel has its own hose and nozzle. There is no button or interface to choose which one you want, you just pick up the correct hose. Most stations tend to have 3 or 4 hoses per pump: regular unleaded, premium unleaded, regular diesel, and premium diesel. Motorway stations sometimes don't have premium unleaded as diesel cars are more common for long distance drivers. Where LPG or Autogas is offered, a dedicated filling area with pump/hose is present. – DizzyFool Mar 16 '17 at 11:11
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    @HariGanti next time you are at the pump, take the hose and try to empty whatever is in the hose before squeezing the trigger. You might be surprised. Also, what about diesel? Does that mean diesel drivers are contaminating with petrol and vice versa? Cos that's pretty flawed. I don't think the regular vs. premium is a big issue but the diesel one definitely is. – DizzyFool Mar 17 '17 at 15:59
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If you fill your vehicle with that little fuel, your only problem is not that the previous user of the fuel pump may have filled with regular gasoline. Another problem is that fuel pump accuracy is not that good with low fuel volumes. You may end up paying for more than what you really filled up your vehicle with.

Apart from the answer by Solar Mike, another possibility is to purchase a 20 liter canister and fill that completely. Then fill your little vehicle from this canister. I guess if your vehicle has only a little more than 3 quarts of gasoline, you will probably be filling it up very often so the convenience of not having to visit the gas station so often may be an important consideration.

However, before purchasing the fuel canister, do check your local laws related to storage of gasoline. You don't want to break the laws, as in the case of a fire, if it is found you had too much gasoline improperly stored, it may be the case that the insurance company won't pay you anything. I suspect a metal canister may be the safest option.

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If you consider this to be such a problem, then you could choose a pump where the last user selected the grade that you want. The actual answer to this question needs someone who builds or services the pumps, which, most probably, is not likely to be found on this forum.

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    How would you know what the last user selected? – cory Mar 16 '17 at 16:24
  • You could watch the other customers.... And there is probably some type of selector - what is it left on? – Solar Mike Mar 16 '17 at 17:18
  • +1: where I live, the pumps show the cost of the last fill-up and the type of the fuel the last customer used. – juhist Mar 17 '17 at 16:14

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