Sounds like you need a new fuel level sender. There are two under the back seat and I suspect one measures the fuel level for the gauge in the instrument cluster and the other is high accuracy for the computer that calculates the distance to empty. I haven't removed the ones in my X, but in other Bimmers the senders use two thin wires and a float to measure fuel level. If the float can't make a clean connection to the wires because of dirt or oxidation, the computer will continually misinterpret the amount of fuel you have left in your tank. I have the same problem with my "distance to empty" calculation on my 300zx turbo as well and I know the fuel level sender is the problem.

Regardless, the engine doesn't stop if the distance to empty reads zero (at least not on my X or my Z), so keep on driving until you can or want to fix the sender.

By the way, I did have both senders changed under warranty in my X around 100,000 kms. The problem I had was that the gauge in my instrument cluster would never show more than 3/4 full even when I crammed 90 liters of fuel in the tank. I think the wires towards the top of the sender must have been dirty so that when the float went to the top, the gauge couldn't "see" that it was there.

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ForbesZ