Cars Easter Egg - Mac and Peterbilt

when Lightning Mcqueen loses Mac (the truck), he goes looking for him. Well, he runs into another truck and yells "Mac", but the truck answers: "I ain't no Mac, I'm a Peterbilt."

This is a reference to the brothers who created Diesel trucks in general. They used to be partners, but then had a tiff and decided to go their separate ways. The two major Diesel trucks are now Mac and Peterbilt.