Originally Posted by andrewwynn

This does not apply to x5. The pump is submerged until there is little enough fuel you could walk home. Approximately 2L or 1/45 of a tank is enough gas to keep the pump submerged. Once a x5 has less than half a tank it always is using the last two gallons of gas.

If you get below about 10 miles to empty there is some chance the pump will start being exposed to air, that being said, the metal shell of the pump is quite thermally conductive and you can worst case drive 20 minutes or so once fuel would get that low. Simply not long enough to cause a problem.

Running out completely is a different story and the manual specifically mentions don't run out completely.

Think of it like this: you are at a bar drinking beer, but out of a shot glass. You have a full stein next to the shot glass: every time you take a sip, the bartender uses the stein to refill the shot.

The shot is your only source of drink and until the stein is empty you have beer always but just out of a tiny glass.

This is exactly how x5 works. There is a tiny (8L ) tank that is all "the engine" knows about. The main tank refills the tiny tank continuously until it (main tank) is empty.

There is no benefit in "having more than 1/8 of a tank" in an x5 from the engine or fuel delivery standpoint no 1950s myth about gas apply to the x5.

There is a very good reason for this design: the "well" in the fuel tank that is the "surge tank" or reserve tank is very deep and narrow: this is so even if the car is at a 37° angle, the pump will stay submerged even with less than 4-5 L of gas! (In fact if you are nose down pitch at say 30-40°, the car will only "see" perhaps 3-4 L of gas because the rest will have spilled out of the 8L surge tank.

As somebody who's experienced fuel starvation with 1/4 tank of gas from parking on a driveway with maybe 5° or slope I really love this system.

Additional benefits include elimination of grime from the bottom of the tank, there simply is none. After 131,000 miles my wife's x5's tank looked 1 day old.

Water sinks. Any water gets in its the first thing pumped out.

Having recently been stranded SPECIFICALLY BECAUSE my wife was brainwashed to "never go below 1/4 tank".

If the siphon fuel pump (that moves the gas from the main tank to the "shot glass") fails, the engine will starve of fuel with somewhere between 1/8 and 1/2 tank of gas.

Since my wife never ever runs her tank low, it masked the fact her fuel pump was failing and left me stranded "with 77 miles to go".

It's important you periodically (I would do it at least quarterly) run the tank down to the low fuel light it's the only way you can verify the siphon pump is working lest you suddenly go from 450-500 mile range to 350-400 miles of range (with no warning).
I wrote the most detailed write up available online on how the x5 fuel delivery system works recently. I plan to add some graphics to describe the system better soon.