Yes, in that sense it is based on consumption. I'm asking the question badly. Let me explain from my airplane days.

It's not about rate of use, but amount of fuel.

Float gauges are not always very accurate. In some cases (e.g. many airplanes) when you put a fuel computer on, if you fill to a known point (e.g. mine had both a "full" and a "to the tabs" which were fairly precise when level) you tell the computer that, and it was far more accurate to know how much fuel was left based on it tracking consumption than on the float guages.

But to do that you must have a known starting point after refueling, which doesn't appear to exist for the X5.

So I guess I'm answering my own question, when it says there are 211 miles to go, it is using the average consumption rate, but dividing that into the float-guauge estimate of amount of fuel.

Or is it not a float?

PS. as to the run-out-of-fuel game, you really should combine it with something like Alligator Alley in florida. Drive around until it says "50 miles to empty" and then get on at the point where it says "next service 50 miles". Unfortunately there are not really any alligators on the road any more, otherwise it would be much more interesting.