I have both a nissan sentra and a nissan xterra and I've noticed this problem with both.
The light to say its empty comes on a good 10L before the tank is officially out of gas. So i'm never using anywhere close to the true amount of gas in the car. With my Xterra the light comes on at about 18L before the tank is empty (80L maximum). Also the Xterra has the DTE (distance to empty) that tells me 50Km to go when I've used about 60L.
Are all cars designed to have a reserve tank of 10 to 15L of fuel, is it safe to run them until theres very little left? and do you do much damage if the tank does run dry?