Ryan Dunn's fatal car crash tied to drinking: How much alcohol does it take?

(CBS/AP) What caused the fiery car crash that killed "Jackass" star Ryan Dunn and his passenger in Pennsylvania on Monday night?

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Speed was one factor. Police said Dunn, 34, may have been driving 140 miles an hour when his Porsche jumped a guardrail, struck a tree, and burst into flames.

But speed wasn't the only factor.

Dunn had been cited for drunk driving previously, and police said his blood alcohol concentration (BAC) at the time of the accident was 0.196 percent - more than twice the legal limit of 0.08.In the four hours leading up to the crash, Dunn reportedly downed two beers and six shots.

How much can people drink before they become drunk?

In part, it depends on gender and weight. Men can drink more than women before becoming impaired, according to Dr. Scott Walters, an alcohol expert at the University of Texas School of Public Health. And, he said, heavy people can drink more than people who weigh less.

But he said the rule of thumb is about one drink per hour. "That means 12-ounce beer, five ounces of wine, or a mixed drink with one shot of 80-proof alcohol," he said. "People think there's a lot of variability in how much people can hold their liquor. In fact, there's more or less a mathematical formula" that determines how quickly the body metabolizes alcohol.

The University of Oklahoma has a handy blood alcohol concentration calculator.

Frank Herron, the general manager of the bar where Dunn had been drinking, said he hadn't appeared drunk.

"He spoke clearly," he said. "He walked clearly. He came in hop, skip, jumping. He left hop, skip, jumping. If these results are true, I'm shocked at it. We were very confident that he had not had that much."