Follow up: it is bad to run a pump dry which on x5 can not happen until you drive about 15-20 miles AFTER DTE reads 0.0, unless your siphon jet has failed. There is no benefit at all for the X5 to have more than 1.4L of gas on the gauge at that point is were any additional fuel spills over from the pump side to the siphon side and provides no additional cooling due to being submerged. So for the X5, it is definitely an "old husband's tale" there is a benefit of having more than a couple gallons of gas in the tank. This only applies to the X5: I can't confirm any other cars that use this system, I've parked on a sloped driveway with 1/4 tank of gas and wasn't able to start my 2002 Ford explorer, most cars have a flat bottom tank with the pump which is not submerged for much of the time.

The X5 tank was designed to keep the pump submerged when going up and down steep terrain but has the side effect of keeping the pump completely submerged down to 0.0 miles to empty and from sucking air for another 10-15 miles past empty.

The X5 thinks it has A 5.0L gas tank: it is continuously refilled so it never drops but once there is less than about 1/3 of a tank the right side will show about 1.4L of gas down to about 7 miles to empty and only once you get closer than that to empty will the fuel pump start to have less fuel above it. You have to get below zero before the pump starts to become uncovered and to about negative 15 DTE before you risk actually running the pump dry.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)