Each race that a car finishes, it gains 1 XP. The amount of XP a car has, determines the Car Class it is in. If a car does not finish a race (due to tire or engine problems, or crashing), it will not gain XP.
XP can also be lost. For every one month period that a car does not participate in a race, it loses 5 XP. When a car is sold, XP and upgrades stay with the car.
Cars join races based on Car Class, which is determined by XP. Only certain classes of cars can race on certain Racetracks. Current Class = Car Class.
Car Class Level
XP Point Range
Factory Class & Current Class
The class in which a car begins is called the Factory Class, while the class it is currently in is called the Current Class. Factory Class is also abbreviated "FC".
Every car contains both of these fields and when it is first minted, the fields will be identical. As the car gains XP, it will move up in classes and that will be reflected by the Current Class.
In order to protect the value of higher classes (prevent dilution by lower classes gaining XP), this distinction was made. While the Current Class determines which Racetracks the car can race on, the Factory Class is used for benefits & perks.
A car with a Factory Class of Rookie and a Current Class of Aero.
Can race on Ho-Pin Tung Circuit (Rank 3) and Petrov Raceway (Rank 4), based on Current Class (see Racetracks)
Can only claim Rookie perks (ex. cannot claim Lootbox drops for Factory Class Aero)
Even though a Current Class Rookie can rise in classes by gaining XP, and one day become a Current Class Legendary, it will not inherit the benefits & perks of a Factory Class Legendary car.