system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it seem like the first half of my gas tank gets more miles than the second?

For example, when the gas gauge is at 1/2 I am at 250 miles.   But when my gas light comes on I'm at 420 with a range of 30.
gpt
Gas tanks are not perfect boxes. They are usually odd shapes so where it is wider, more gas is needed to be used to drop the level compared to where it is narrower. 

There is a float the determines the fuel level but you can fill the tank beyond that level so the first bit of gas used doesn't even make the gauge budge.