Another reason the gauge drops faster as the tank empties is because the sender doesnt actually measure the number of gallons in the tank, but the level the fuel is at. If you look at a mustang tank from the side, it is sort of V shaped, the front is flat, but the rear slopes down. Therefore if the float is near the top of the tank, there would be more gas to burn before the level dropped compared to the bottom of the V.
-----------
|---------| Full
|---------/ 3/4
|--------/ 1/2
|-------/ 1/4
-------- empty
Does that make sense to anyone but me?
Tim