I have a 2000 S80, occasionally the "Miles to Empty" indicator will go up and I mean waaay up.
Once I watch the indication increase to 585 Miles (If only it were true...). It will eventually drop back down to some semblance of reality, sometime in a few minutes other times it kept creepin up for two days. Any ideas why this might be happening or is this just one of the 00' quirks that everyone keeps refering to.

 

The "miles to empty" function is just a mathematical process which takes your instantaneous mpg and multiplies it by the amount of gallons you have left. Therefore, say you are "coasting" down a long hill for a mile or so...your instantaneous mpg will read about 80 mpg because you are barely using any gas at all to "idle" down the hill. Multiply 80 mpg times how much gas you have left in your tank...say 8.0 gallons...and that will cause your "miles to empty" alert to read about 640 miles. we know this is bogus, simply because there aren't any hills in the world that are 640 miles long...but if there were, it would take about 8 gallons to reach the bottom.
Basically, the "miles to empty" function is constantly updating itself in relation to the instantaneous mpg so that it constantly gives an accurate reading of how many miles you have till its time to fill up again.
It could be a quirk, but my "miles to empty" occasionally gets up near 600 too...however, I find that i normally dont get much more than 450 miles to a tank...that turbo is just too much fun not to use!

 

You might also have a sending unit going bad or the float hanging up on the wires in the tank.