Demonstrating once again the unreliability of tracking for family films — and the power of the Pixar brand — Disney’s “Cars 2” revved up Stateside box office with an exceptional $68 million opening, according to the Mouse’s domestic weekend estimates.

Cars 2” added an estimated $42.9 million in overseas grosses through Sunday, which brings the film’s total worldwide debut to $110.9 million.

Also firing on all cylinders at the weekend’s domestic B.O., Sony’s R-rated comedy “Bad Teacher” exceeded industry expectations with a projected debut at $31 million. Pic chalked up a particularly strong overseas weekend take for an American comedy, with an estimated $12.9 million from mostly smaller markets and a second outing in the U.K. Worldwide cume on “Bad Teacher,” which cost just under $20 million, is already at $49.3 million.

Domestically, “Cars 2” and “Bad Teacher” helped fuel another weekend-to-weekend improvement vs. 2010, with overall B.O. totals beating the same frame last year by approximately 9%. That should be a welcome relief for bizzers, especially those at Disney, since the fate of “Cars 2” this weekend was never certain.

The Pixar toon, which continues the animation house’s perfect track record of No. 1 domestic opening perfs (at 12), had most B.O. observers confounded prior to the weekend.

In fact, notoriously hard-to-predict tracking on family films created a wide range of opening expectations, starting at the high-$40 million mark. But Friday’s $25.7 million perf — a surprisingly noteworthy start for a family pic that appeals mostly to tykes — had bizzers revising their weekend predictions at around $60 million.

“Cars 2,” however, did have an unimpressive weekend at 3D locations, taking in just 40% of its opening from 3D.

Warner Bros.’ 3D holdover “Green Lantern” saw a big drop in its second frame, down 65% for an estimated $18.4 million. “Green Lantern,” which fell far more than both “X-Men: First Class” (56%) and “Thor” (47%) did in their soph seshes, has cumed $89.3 million domestically. Pic’s tally from 16 markets outside the U.S. reached $29.4 million through Sunday.

The frame’s other soph player, 20th Century Fox’s “Mr. Popper’s Penguins,” did better, down 44% for an estimated $10.3 million (even with “Cars 2” hogging a sizable share of the family market). “Penguins” has grossed $39.4 million, with an additional $4.6 million from 16 overseas territories.

Pixar-effect fuels ‘Cars’

Disney distribution topper Chuck Viane said “Cars 2” benefited from Pixar’s loyal fanbase, even going so far as to describe the toon as a fanboy pic driven by “animation fans who can’t wait to see the latest Pixar film.”

“It’s pure trust of the label,” Viane said of the toon’s Pixar lift.

Indeed, Pixar films usually show exceptionally strong legs. The original “Cars” saw a four-times multiple, cuming $244.1 million after its $60 million debut on June 9, 2006.

In fact, “Cars 2” falls near the top of Pixar openings (more than half of the label’s 12 films, including “Cars 2,” opened between $60 million and $70 million). “Toy Story 3” ranks as Pixar’s best with its $110 million opening and a domestic cume of $415 million.

Adding to the long-term prospects for “Cars 2”: The toon received an A- CinemaScore rating and saw a relatively even split between men (53%) and women (47%).

Pic also played best with audiences under 25, which accounted for 56% of the debut demo, while auds 12 and under made up 36% of the opening.

But “Cars 2” has stiff competish looming, led by Paramount’s “Transformers: Dark of the Moon,” then Warner’s “Harry Potter and the Deathly Hallows: Part 2.” Imax, which has exceptional pre-sales for both “Transformers” and “Harry Potter,” contributed $3.25 million this weekend to “Cars 2” grosses from almost 150 worldwide locations.

With Sony’s “Bad Teacher,” the opening aud was more skewed than with “Cars 2.” Femmes repped 63% of the laffer’s bow — typical of most comedies — while 57% of auds were over 25.

“Bad Teacher” was likely helped by the rabid popularity of a similar R-rated comedy, “Bridesmaids,” which dropped just 24% in its seventh frame.

Rory Bruer, prexy of worldwide distribution at Sony, nevertheless said those two pics and “The Hangover Part II” each offer auds something different. “Each film really worked based on its own merit,” Bruer said. “With ‘Bad Teacher,’ you’ve got to give a ton of credit to Cameron (Diaz).”

It’s uncertain whether “Bad Teacher,” with its poor C+ CinemaScore rating, will register with auds like “Bridesmaids” or “Hangover II.”

Indie high, low road

In its sixth week (and third in wide release), Sony Pictures Classics’ Woody Allen-helmed “Midnight in Paris” scored its fifth consecutive top-10 perf with an estimated $4.5 million, bringing its U.S. cume to $28.6 million — behind only Allen’s top two pics, “Hannah and Her Sisters” ($40.1 million) and “Annie Hall” ($38.3 million). “Paris” could surpass those films given its recent playability. (Pic lost 87 locations vs. last weekend for a total 951 but dropped only 8%.)

Meanwhile, Summit immigration drama “A Better Life” got off to an OK start at two locations each in New York and L.A. Pic, which reportedly saw a healthy demo mix, tallied a total $60,000 for a per-screen average of $15,000.

Other notable specialty perfs included Italo music pic “Passione,” from helmer-star John Turturro, which took $17,294 at one location. Earning less, but debuting at 24 locations, docu “Conan O’Brien Can’t Stop” (both pics are distribbed by Abramorama) earned a weekend total of $104,833 for a not-so-hot per-screen average of $4,193.

Oscilloscope’s docu “If a Tree Falls: The Story of the Earth Liberation Front” not surprisingly did better at fewer locations (just two), with a $7,050 per-screen average; Sundance Selects’ docu “Buck” expanded to 54 engagements, up from four, averaging a soph-sesh per of $5,500 and hitting a cume of $395,000.