NEW ORLEANS — BP's massive oil spill today became the largest ever in the Gulf of Mexico based on the highest of the federal government's estimates, an ominous record that underscores the oil giant's dire need to halt the gusher.

The oil that's spewed for two and a half months from a blown-out well a mile under the sea hit the 140.6 million gallon mark, eclipsing the record-setting, 140-million-gallon Ixtoc I spill off Mexico's coast from 1979 to 1980. Even by the lower end of the government's estimates, at least 71.7 million gallons are in the Gulf.

The growing total is crucial to track, in part because London-based BP PLC is likely to be fined per gallon spilled, said Larry McKinney, director of Texas A&M University at Corpus Christi's Gulf of Mexico research institute.

"It's an important number to know because it has an impact on restoration and recovery," McKinney said.

The oil calculation is based on the higher end of the government's range of barrels leaked per day, minus the amount BP says it has collected from the blown-out well using two containment systems.