How does the fuel value get decremented in this example?

Welcome to the Cesium community @Jayson_Thomas!

The fuel here is part of the CZML file, which has defined points in time where its value changes, like here: https://github.com/CesiumGS/cesium/blob/4603fd33e40db78c0baf9531ad301f64d9d63049/Apps/SampleData/MultipartVehicle_part1.czml#L52-L58

What kind of project are you working on?