on my mazda the gauge never moves for the first 30 miles, then once it gets to 1/2 it falls like a stone, very quickly. SO basically i get 250 miles on the first "1/2" and 50 miles on the 2nd "1/2". You just learn your vehicle.
Not that this matters, but keep in mind the shape of some car tanks. They tend to be wider at the top and get smaller near the bottom.
So from a physical height centerline there is "usually" more volume in the top centerline up portion of the tank than the bottom portion of the tank.
