What was the first Pixar short film set in the world of Cars called?

The first Pixar short film set in the world of Cars was Mater and the Ghostlight.

Mater and the Ghostlight is a 2006 short included as a bonus feature on home-video releases of Cars. It follows Mater after the residents of Radiator Springs play a practical joke on him by telling a frightening story about a mysterious blue light. The short expands Mater's personality and uses the town's nighttime setting for a playful ghost story.

The short was directed by John Lasseter and Dan Scanlon. It predates the Cars Toons television shorts that began with the series Mater's Tall Tales. Those later episodes place Mater in exaggerated stories involving professions, adventures, and imagined exploits, whereas Mater and the Ghostlight is closely tied to the first feature's Radiator Springs setting.

The title is sometimes confused with Cars Toons episode names such as Rescue Squad Mater or Heavy Metal Mater. Those are later shorts, not the first Cars-related Pixar short. Mater and the Ghostlight also appeared with a song and story built around the fictional legend of the ghostlight.