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I've almost always filled when there is about 1/8 of a tank and have had good results (check my fuel-log). There's a study by Daox floating around here that demonstated optimum FE at different tank levels. The weight could provide more momentum on coasting, while a lighter tank reduces acceleration load. At the least, I prefer the accuracy of filling when near-empty.
Modern vehicles have an on-board vapor recovery system to prevent fuel vapor from evaporating into the atmosphere and creating "Ozone Alert Days" from the fuel creating inhalation hazards. This may be the "sucking sound" experienced. Some municipalities require the same through the fuel pump itself -- basically a return line at the end the spout returns vapor to the station's supply tank. Between fills, fuel-injected vehicles have a return line to send excess fuel back to the fuel tank from the fuel rail. This is over-simplified, but you get the idea.
In conclusion, I would like to see some hard data (at least 3 tests, repeatable) on the water vapor theory. I admit it would be difficult, but possible to establish significance.
RH77
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“If we knew what we were doing, it wouldn't be called research” ― Albert Einstein
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