Mods are the ability to change the content in a video game, out of the Cars games, the PC versions of Cars: Mater-National Championship (which has open files that can very easily be changed) and Cars: The Video Game (which is much harder to mod, read this to learn how) are confirmed moddable. Certain pictures found also suggest that the first game on the Xbox 360 is moddable, but this is yet to be confirmed true. This is a list of various things that can be done using simple mods. More about modding Cars games can be read on Cars Video Game Modding Wiki.
Minor Mods Edit
Each track directory contains a file called .aty, when opened using Notepad, the player can edit it, changing the character codenames listed in it will change the car that appears as that opponent (the player's car cannot be changed). A line called "ALODLevel" will determine which cars are faster or slower, although this mechanic is not fully understood yet. Changing "StartLineOrder" determines which one of 20 starting positions they will start in. More cars can also be added/removed from the race, however, only 20 cars can appear in one race, adding a 21st racer will crash the game.
Character voice clips and music can be changed by obtaining audio files, and copy/pasting them into the game's C/Audio/Music or E/Audio files, then editing the game's "Music.pb" and "common.txt" or the character's .aud file and list that they will play, you can also change which music plays in which event type. Also know that audio files must be in .ogg format for them to work.
Major Mods Edit
Add new Paint Jobs Edit
By taking a character's texture file and converting to a simple format such as .png, and using a texture editor, one can make new paint jobs for characters, once they retexture it, they must convert it back to .dds and add certain listings to the VehicleInfo.dat file. They can then select the paintjob for the character in the game.
Play as NPCs Edit
By adding playable animation files and listings to the VehicleInfo.dat and EventInfo.dat, one can make new characters playable. Some characters have some playable data by default, but as of now, every NPC in the game has successfully been made playable. By porting, one can also add characters that are not in the original game at all.
Community-Made Projects Edit
This mod was made by some people in order to add some things from Cars: The Video Game and Cars: Race-O-Rama to Cars: Mater-National Championship on PC. It allows players to use many new characters and some paint jobs for them, along with some tracks from the other two games. The last official version of this mod was v1.9.1, released in August 2015.
This mod was to be made by the same makers of Cars: Hi-Octane Edition for the Xbox 360 version of Cars: The Video Game. It would have allowed the players to race with the unplayable characters who appear in this game along with some cars who do not appear in this game at all, but the project was abandoned.