A Bermuda triangle in the gas tank?

["Salah Khan" <salah.khan@gte.net>]

Hi all,
On my way to the airport last night, I ran out of gas. Had to walk three
miles to the closest phone, etc, etc. Anyway, the V8 is supposed to hold 21+
gallons of gas. But after 18.5 it ran dry. Yes, the computer warning sign
had come on, but I was relying on the gas gauge which indicated I still had
at least a couple of gallons left.  I have never been able to put more than
18.5 gallons of gas in the car. So what happens to the last 2.5-3 gallons?
Does it disappear into thin air (that is, is there an air pocket or
something in the system/tank)? And what might be the fix?
If anyone has experience with this, I'd appreciate hearing from you.
Perplexed,
Salah