As the mother of a 3-year-old, I have the research to back up this statement: “Cars” holds up pretty well to multiple viewings. And by “multiple” I mean “at least 50.” “Cars 2,” out today, probably won’t.

“Cars” is the story of, in the words of my son, “a real fast race car” named Lightning McQueen, who learns a valuable lesson when he ends up in a small town. One of his mentors on his journey is a rusty tow truck named Mater. In “Cars 2,” Mater is the main character — and that’s the problem.

In the first film, where McQueen is a nuanced, well-rounded character, Mater mostly serves as a one-note foil. He’s fun to watch and is absolutely necessary, but his major function is to show McQueen that you can be happy even if you’re not a real fast race car. He’s like the gravedigger in “Hamlet” — he’s well-written, he’s enjoyable to watch, and he serves a metaphorical purpose, but, well, there’s a reason there’s no play called “Gravedigger.”

Of course, young fans will love “Cars 2,” and it will probably be a financial success, even before factoring in the Lightning McQueen toys and the clothes and the sheets and the insulin and whatever else they’re selling.

But the mistake Pixar made in this, its weakest film by far, was thinking that a tow truck could pull a story along.

Photos courtesy of Disney/Pixar