"Our tracking was stalled toward the end," said Dan Fellman, president of Warner Bros. theatrical distribution, about market surveys before the "Speed Racer" opening.
Fellman added "Speed Racer" would have difficulty recouping its $120 million budget.
"Unfortunately it didn't perform to our expectations," he said.
"Speed Racer" was panned by many film critics and was made by the Wachowski brothers, best known for the "Matrix" franchise.
"What Happens in Vegas," a romantic comedy that cost $35 million and stars Cameron Diaz and Ashton Kutcher, shot the gap between the action flicks, said Chris Aronson, senior vice president at 20th Century Fox.
"We were sandwiched between the comic book movie of 'Iron Man' and 'Speed Racer,"' Aronson said. "We come along with a character-driven movie in what is now considered summer and it's just a great result."
Revenue for Marvel Studios' "Iron Man" slipped just 49 percent in its second week in theaters, a respectable showing considering its massive opening, said Paul Dergarabedian, president of tracking firm Media By Numbers LLC.
"'Iron Man' is basically pulling everybody's audience. It's really dominating the marketplace," he said.
Five of the top 10 grossing movies were comedies, including Sony Pictures' Patrick Dempsey-led film, "Made of Honor," which took in $7.6 million in its second week, for fourth place.
The weekend's total domestic box office gross hit $128 million, up 21 percent from last year. But year-to-date revenue was still down 2.5 percent at $2.93 billion, while attendance was off 5.3 percent.
Over the next few weeks, several big movies will take a shot at knocking "Iron Man" off its perch, starting with "The Chronicles of Narnia: Prince Caspian," opening May 16, and "Indiana Jones and the Kingdom of the Crystal Skull" on May 22.
Estimated ticket sales for Friday through Sunday at U.S. and Canadian theaters, according to Media By Numbers LLC. Final figures will be released Monday.
1. "Iron Man," $50.5 million.
2. "Speed Racer," $20.2 million.
3. "What Happens in Vegas," $20 million.
4. "Made of Honor," $7.6 million.
5. "Baby Mama," $5.8 million.
6. "Forgetting Sarah Marshall," $3.8 million.
7. "Harold & Kumar Escape from Guantanamo Bay," $3.2 million.
8. "The Forbidden Kingdom," $1.9 million.
9. "Nim's Island," $1.3 million.
10. "Redbelt," $1.1 million.