Rumors are a very common thing in video games, and the Cars series is no exception. This article goes over a few.
Piston Cup Racers
Not much about this rumor is known, other than that some thought there was a secret way to play as the 18 NPC Piston Cup Racers in Cars: The Video Game. This rumor was eventually solved as mods for the first game began appearing, it was false.
ENGINPWR
In the intro of Cars: Mater-National Championship, a quick clip of Snot Rod in player's perspective appears, so many have thought Snot Rod was somehow playable, a rumor soon sparked when someone posted a picture of Snot Rod as the player on Radiator Springs Circuit, and it was soon decided that he was playable only on the PC version of the game by typing "ENGINPWR" in the Cheat Codes. The rumor was later proven false and that the picture was a mod.
Gallery
The intro clip that sparked ENGINPWR.